Soft, fluffy squares of vanilla butter cake, flavored with lemon and berries and slathered with sweet-tart lemon icing. Sprinkles recommended. Plate and fork not required.
For the cake:
- 2 1/2 cups all-purpose flour
- 3 1/2 tsp baking powder
- 1 1/4 tsp table salt (2 tsp kosher salt)
- 12 ounces berries fresh or frozen (*See note)
- 2 tbsp cornstarch
- 1 2/3 cup granulated sugar, divided
- 1/3 cup vegetable shortening
- 1/3 cup (5 1/3 tbsp) salted butter, at room temperature
- 2 tbsp lemon zest (From 1–2 lemons, depending on their size. Juice lemons for icing.)
- 1 tsp lemon extract
- 2 tsp pure vanilla extract
- 2 large eggs, at room temperature
- 3 large egg whites, at room temperature (How to quickly bring eggs to room temperature)
- 1 1/4 cup milk, whole or 2% (how to quickly bring milk to room temperature)
For the lemon icing:
- 6 tbsp salted butter, at room temperature
- 3 cups powdered sugar (confectioners sugar)
- 6–7 tbsp freshly squeezed lemon juice
- 1 tsp pure vanilla extract
- 1 tsp lemon extract
- 1/8 tsp table salt (1/4 tsp kosher salt)
Make the cake:
- Preheat your oven to 350 degrees. Use either vegetable shortening or butter to grease and flour a 9×13-inch rectangle baking dish. (How to grease and flour pans.)
- Add the flour, baking powder, and salt to a bowl and stir with a wire whisk to combine.
- Add the berries to a separate bowl and toss with the cornstarch. Set aside.
- Add 1 1/3 cup of the granulated sugar, the shortening, and butter to the bowl of a stand mixer fitted with the whisk attachment. Beat on high speed until very light and fluffy, about 6 minutes, stopping the mixer a few times to scrape down the sides of the bowl with a rubber spatula.
- Separate the two whole eggs, adding the egg yolks to the butter and shortening mixture and the whites to a separate bowl. Beat the egg yolks into the butter shortening mixture until well blended, about 1 minute. Add the zest, lemon extract, and vanilla extract and beat for another 30 seconds to mix.
- Fit the mixer with the paddle attachment. Add the flour mixture and milk in alternating additions, beginning and ending with the flour (1/3 of the flour, 1/2 of the milk, 1/3 of the flour, 1/2 of the milk, 1/3 of the flour). After each addition, beat the batter on low speed just until barely incorporated. Scrape down the sides of the bowl before each new addition.
- Add the 3 remaining egg whites to the other two egg whites and beat with an electric mixer until doubled in volume and frothy. With the mixer running, slowly sprinkle in the remaining 1/3 cup of sugar. Beat until medium peaks form; if you lift the beater from the egg whites, the peak that forms will hold its shape, but fold over on itself slightly. (See images and video above for more information.)
- Gently stir the berries into the batter, mixing just long enough to distribute them. Fold the beaten egg whites into the batter as gently as you can, mixing just until distributed throughout the batter. Do not over mix. You should still be able to see a few streaks of egg white in the batter.
- Scoop the batter into the prepared pan, smoothing it into an even layer. Bake for 60-65 minutes, until the sides of the cake are pulling away from the side of the pan, the cake does not jiggle at all if you move the pan, and a toothpick inserted in the center of the cake comes out without any evidence of raw batter. (*See note about what to do if the top of the cake is getting too brown.)
- Remove the cake to a wire rack and allow to cool completely in the pan before icing.
Make the lemon icing:
- Add all the icing ingredients to a large mixing bowl and beat with an electric mixer until completely smooth, about 3-4 minutes. Adjust the amount of lemon juice and powdered sugar as necessary to achieve a spreadable consistency. Icing should be the consistency of yogurt.
- Spread the icing over the cake and allow it to set before cutting into squares, about 2 hours at room temperature or 1 hour in the refrigerator.
- Top with sprinkles if desired, cut into squares and serve. Cake will keep covered at room temperature for up to 2 days.
- What kind of berries should you use? I really love using raspberries in this cake but blueberries and blackberries will also work well. You can also use a combination of berries. If you use strawberries, cut them into pieces that are about the size of a blueberry or raspberry.
- What to do if the top of your cake is getting too brown: Because it takes about an hour to bake this cake, the top can get overly brown before the center is baked through. It’s ok if the top of the cake is golden brown, but if the top of your cake starts to look too dark, simply cover it loosely with a sheet of aluminum foil.