A spin-off of the traditional pineapple upside-down cake, we use seasonally and regionally available strawberries and rhubarb to create a mouth-watering and gut-healthy treat where coconut sugar caramelizes in butter, and the strawberries pair perfectly with tart rhubarb. Plated and decorated with love, this Sourdough Strawberry Rhubarb Upside-Down Cake promises to bring joy to all.

sourdough strawberry rhubarb upside down cake sitting on a cutting board with fresh strawberries and rhubarb stalk over a tea towel on the counter top

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Below is the quick & printable version of this recipe. For the more detailed version, sourcing quality ingredients, and frequently asked questions, scroll below. Enjoy!

Sourdough Strawberry Rhubarb Upside-Down Cake

A seasonal upside-down cake, using spring strawberries and rhubarb caramelized in maple sugar and butter to create a gut-friendly indulgence.

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Servings 8

Ingredients

1 Cup Rhubarb About 2 stalks, sliced ½" thick
1 Cup Strawberries Stems trimmed, halved
1/4 Cup Butter Half Stick
1 Shot Dark Rum
3 1/2 Cups Sourdough Starter active or discard
1 Egg
1 tsp. Salt
1/4 tsp. Ground Cardamom
1/2 Cup Maple Sugar
1 stick or 1/2 Cup Butter
1/4 Cup Milk the amount you use may vary depending on the thickness of your sourdough starter.
1-1/2 tsp. Baking Powder

Instructions

In a small saucepan or skillet place one stick of butter over medium-low heat to brown the butter.
When the butter has browned, set aside to cool for 10 minutes.
Preheat the oven to 400 degrees Fahrenheit, and preheat the cast iron skillet on the stovetop over low heat.
Prepare the fruit by chopping the rhubarb into 1/2" slices removing the stems from the strawberries, and cutting them in half.
Get 1/4 cup of butter in the cast iron skillet and turn the heat up to medium-low. When butter has fully melted, stir in brown coconut sugar, and a shot of dark rum. Mix with a whisk constantly for one minute while the sugar dissolves and the mixture bubbles to create a caramelized pan of goodness.
Remove from the heat and spread rhubarb and strawberry slices in a layer over the sugar mixture, completely covering the mixture. Set aside.
In a mixing bowl combine sourdough starter, cardamom, salt, maple sugar, and an egg until fully incorporated.
Next, add the cooled brown butter, and 1/4 cup of cold milk.
Add Baking Powder, and mix thoroughly.
Pour the batter over top of the rhubarb and strawberry slices in the skillet; spread evenly to cover.
Bake in preheated oven until the cake begins to bubble around the edges and the top is golden brown. Add a toothpick in the center to see if it comes out clean. 30-35 minutes.
Cool inside the skillet on the counter for 30 minutes.
After the cake has cooled, place a cutting board on top of the skillet invert the cake by flipping it over onto the cutting board. The skillet should come out clean.
Transfer the cake onto a cake plate or pedestal and decorate.
Bon Appétite!

Notes

You want the batter to be the thickness of the pancake batter. You can adjust the thickness of the batter with a little more or a little less milk. It all depends on the consistency of your starter.
If the cake doesn’t immediately come out of the skillet, run a knife around the edge of the pan to loosen it, and try again.
I used strawberries and rhubarb, but this cake works well with many different seasonal berries and stone fruits.

Equipment you may need:

1- 9″ Cast Iron Skillet
1- small skillet or saucepan
1- medium-sized mixing bowl
Measuring Cups & spoons
Whisk
Rubber Spatula
Shot Glass

Ingredients you need:

For Strawberry Rhubarb Mixture:

1 Cup Strawberries- stems removed & halved. About 23 freshly picked spring strawberries
1 Cup Rhubarb- about 2 stalks, chopped into 1/2″ slices,
1/2 stick of butter or 1/4 cup- preferably fresh raw or cultured butter
3/4 Cup of Brown Coconut Sugar
Shot of Dark Rum- I used homemade vanilla-infused rum aka Vanilla Extract

For Batter:

3 1/2 Cups Sourdough Starter– active or discard; this recipe isn’t fussy
1 Egg- Preferably from a happy chicken
1 tsp. Salt- Preferably sea salt or Himalayan pink salt for high mineral content.
1/4 tsp. Ground Cardamom
1/2 Cup Maple Sugar
1 stick or 1/2 Cup Butter– preferably fresh raw or cultured butter
1/4 Cup Milk- preferably raw milk, the amount you use may vary depending on the thickness of your sourdough starter.
1-1/2 tsp. Baking Powder

How to make Sourdough Strawberry Rhubarb Upside-Down Cake:

Step One: Brown Butter

In a small saucepan or skillet place one stick of butter over medium-low heat to make brown butter. When the butter has browned, set aside to cool for 10 minutes.

various bowls of coconut brown sugar, freshly sliced rhubarb and strawberries, a stick of butter and fresh rhubarb stalk and whole strawberries sitting over a tea towel on the counter top to make strawberry rhubarb upside down cake

Step Two: Preheat & Prepare Fruit

Preheat the oven to 400 degrees Fahrenheit, and preheat the cast iron skillet on the stovetop over low heat.

While the oven and skillet are preheating, prepare the fruit by chopping the rhubarb into 1/2″ slices removing the stems from the strawberries, and cutting them in half.

Step Three: Mix 1st Fruit Ingredients

coconut brown sugar, butter and rum bubbling in a 9 inch cat iron skillet, with a bowl of fruit and fresh strawberries and rhubarb over a tea towel to make sourdough strawberry rhubarb upside-down cake.

Get 1/4 cup of butter in the cast iron skillet and turn the heat to medium-low. Stir in brown coconut sugar, and a shot of dark rum. Mix with a whisk constantly for a minute while the sugar dissolves and the mixture bubbles to create a caramelized pan of goodness.

sliced strawberries and rhubarb over carmalized mixture in a cast iron skillet on a tea towel next to whole strawberries and rhubarn stalk on the counter top to make sourdough strawberry rhubarb upside down cake

Remove from the heat and spread rhubarb and strawberry slices in a layer over the sugar mixture, completely covering the mixture. Set aside.

Step 4: Prepare Batter

In a mixing bowl combine sourdough starter, cardamom, salt, maple sugar, and an egg until fully incorporated.

Next, add the cooled brown butter, and 1/4 cup of cold milk. You can adjust the thickness of the batter with a little more or a little less milk. It all depends on the consistency of your starter. You want the batter to be the thickness of pancake or Dutch baby batter.

Lastly, add the baking powder and mix thoroughly.

Step 5: Pour batter and bake

sourdough batter evenly spread over fruit mixture on a tea towel next to whole strawberries and rhubarb stalk on the counter top to make sourdough strawberry rhubarb upside down cake

Pour the batter over top of the rhubarb and strawberry slices in the skillet; spread evenly to cover.

baked sourdough batter evenly spread over fruit mixture on a tea towel next to whole strawberries and rhubarb stalk on the counter top to make sourdough strawberry rhubarb upside down cake

Bake in preheated oven until the cake begins to bubble around the edges and the top is golden brown. Add a toothpick in the center to see if it comes out clean. 30-35 minutes.

Cool inside the skillet on the counter for 30 minutes.

Step 6: Flip the Cake and Decorate

sourdough strawberry rhubarb upside down cake sitting on a cutting board over a tea towel next to whole strawberries and rhubarb stalk.

After the cake has cooled, place a cutting board on top of the skillet and quickly and confidently invert the cake by flipping it over onto the cutting board. It should come out clean.

sourdough strawberry rhubarb upside down cake decorated with spring flowers and powdered sugar sitting on a cake stand

From there, transfer the cake onto a cake plate or pedestal and decorate it to your liking.

Bon Appétite!

Frequently Asked Questions:

1. What is the point of browned butter?

brown butter in a skillet pan to make a sourdough strawberry rhubarb upside down cake

Brown butter is also known as Beurre noisette; in French, this translates to, butter browned to the color of noisette or “hazelnuts.” Beurre Noisette gives a richer more delicious flavor than melted butter.

As soon as your butter is melted, allow it to turn golden, and stir constantly to disperse the nutty flavor. It should take no longer than 5 minutes. Watch closely as it turns from golden brown to burnt quickly. The brown bits at the bottom are a good thing, don’t leave those out when you’re transferring the brown butter to your dish!

2. Is maple sugar better than sugar?

close up of coconut sugar on a sourdough strawberry rhubarb upside down cake

With its high mineral content and depth of flavor, maple sugar is the better option over white sugar for its high nutritive value and less blood sugar spikes. Maple sugar is particularly rich in potassium and calcium, with trace minerals including magnesium, manganese, and phosphorus. And it’s easy to swap out and use because it dissolves just like white sugar, and is 1-1 in any recipe.

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