Step 1

Preheat your oven to 350 degrees F (176 degrees C). Use either vegetable shortening or butter to grease and flour a 9x13-inch rectangle baking dish. (How to grease and flour pans.)

Step 2

Add the flour, baking powder, and salt to a bowl and stir with a wire whisk to combine.

Step 3

Add the berries to a separate bowl and toss with the cornstarch. Set aside.

Step 4

Beat 1 1/3 cup of the granulated sugar, the shortening, and butter with an electric mixer on medium high speed until very light and fluffy, about 6 minutes. Stop the mixer every once in a while to scrape down the sides of the bowl with a rubber spatula.

Step 5

Separate the two whole eggs (here's how to separate eggs), adding the egg yolks to the butter and shortening mixture and the whites to a separate bowl.

Step 6

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.

Step 7

Add the flour mixture and milk to the batter 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.

Step 8

Add the 3 remaining egg whites to the other 2 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.)

Step 9

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.

Step 10

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 below for what to do if the top of the cake is getting too brown.)

Step 11

Remove the cake to a wire rack and allow to cool completely in the pan before icing.

Step 12

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.

Step 13

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.

Step 14

Top with sprinkles if desired, cut into squares and serve. Cake will keep covered at room temperature for up to 2 days.