Sourdough Lemon Sheet Cake with Buttercream

I’m obsessed with lemon everything, but this is one of my favorite desserts of all. The secret is heating the milk with butter and oil before adding it to the batter, which creates this amazing texture that’s part sheet cake, part sponge cake. A from-scratch buttercream with plenty of lemon zest makes it even better I bake it in a 9×13 because it’s so much easier to serve and take anywhere (hello spring potlucks and parties.) No one will ever guess there’s sourdough starter hiding in there unless you tell them. They’ll just know it’s the most delicious, moist lemon cake they’ve ever had. 

When the weather starts getting warmer, I want desserts that taste bright and fresh instead of the heavy chocolate and spice cakes I’ve been making all winter. This lemon sheet cake is one of the best desserts to use your sourdough starter when you’re ready for something lighter. The tart lemon flavor wakes up your taste buds after months of comfort food. Plus, it’s easy to serve outside if you’re finally getting to eat on the porch again. 

What You’ll Need

This recipe is just simple pantry ingredients and plenty of lemons!

Cake

• 2 cups all-purpose flour
• 1 teaspoon salt
• 1½ teaspoons baking powder
• ½ teaspoon baking soda
• 4 large eggs
• 1¾ cups sugar
• ¾ cup sourdough discard (100% hydration, at room temp) this should be equal parts flour and water by weight. If your starter is thicker or thinner, you might need to adjust your liquids slightly
• ¾ cup milk
• ¼ cup butter (room temperature, cut into tablespoon pieces)
• ⅓ cup vegetable oil
• 2 tablespoons lemon zest
• 1½ tablespoons lemon juice
• 2-3 drops yellow food coloring (optional)

Lemon Buttercream

• 1 cup unsalted butter (softened, 2 sticks)
• 4 cups powdered sugar
• 1½ tablespoons lemon zest
• 2 tablespoons fresh lemon juice
• 2 tablespoons heavy cream
• Pinch salt
• Additional lemon slices or zest for garnish

Step by Step

The method here is a little different. I heat up the milk and butter for a super moist texture, oe of my favorite cake tricks. The end result is light but still sturdy enough for a layer of frosting.

Getting Started 

Preheat your oven to 325°F and grease that 13×9 pan really well – I use cooking spray and make sure I get the corners.

The lower temperature is key here because we’re dealing with a lot of sugar and we don’t want the edges to get dark before the center bakes through.

Mix Your Dry Ingredients 

Whisk together the flour, salt, baking powder, and baking soda in a medium bowl.

I like to use a whisk instead of just stirring because it gets everything evenly distributed and adds a little air. Set this aside, we’ll need it soon.

Beat the Eggs and Sugar 

This is where the magic happens. Beat those eggs and sugar for a full 2-3 minutes until the mixture is pale and fluffy. It should almost double in volume. Don’t rush this step.

The air you’re beating in now is what gives the cake its light texture. If you’re using a hand mixer, it might take closer to 4 minutes.

Add the Sourdough Starter 

Gently fold in the sourdough starter until just combined. Don’t worry if it looks a little lumpy at first.

Combine Wet and Dry 

Add your flour mixture to the egg mixture and beat until just incorporated. Scrape down the sides of the bowl to make sure everything gets mixed in.

The key word here is “just”: once you don’t see streaks of flour, stop mixing. Overmixing develops the gluten and makes your cake tough.

Heat the Milk Mixture 

In a small saucepan, heat the milk, vegetable oil, and butter pieces until it just starts to simmer around the edges.

You’ll see little bubbles forming but it shouldn’t be a rolling boil. This warm mixture is what creates that unique texture.

Fold Everything Together 

Here’s where you need to work quickly but gently. Pour the warm milk mixture into your batter and fold it in with a spatula. Then stir in the lemon juice, lemon zest, and food coloring if you’re using it.

The batter will be thinner than typical cake batter. That’s normal!

Bake and Test 

Pour the batter into your prepared pan and get it in the oven. It’ll bake for 30-35 minutes, but start checking at 25 minutes with a toothpick.

The top might look slightly moist and spongy when it’s done. That’s normal. Don’t overbake it or you’ll lose that tender crumb.

Cool Completely 

I know it’s tempting, but let this cake cool completely in the pan before you even think about frosting.

If you frost it while it’s even slightly warm, your beautiful buttercream will melt and slide right off

Make the Buttercream 

Make sure your butter is truly softened; it should give when you press it but not be melty. Beat everything together, adding the cream gradually until you get the consistency you want.

If it’s too thick, add more cream a teaspoon at a time. Too thin? Add more powdered sugar.

Assemble and Serve 

Spread that buttercream right over the cake in the pan. I like to use an offset spatula, but a regular butter knife works fine too.

Top with thin lemon slices or more zest if you want it to look fancy. Cut right in the pan and serve.

Storage Instructions

Cover the pan with plastic wrap or pop slices into an airtight container in the fridge – it’ll stay good for about 5 days.

Want to freeze some? Wrap individual slices in plastic wrap and they’ll keep for up to 3 months. Just let frozen slices sit on the counter for 30 minutes or so before you eat them.

Questions and Troubleshooting

Printable Recipe