CAKE: Position a rack in the center of the oven and preheat the oven to 350ºF. Generously grease a 12x17 inch half sheet pan. Set aside.

Place a sieve over a large bowl. Add the flour, cornstarch, and baking soda and sift. Set aside. In a 2 cup measuring cup, combine the buttermilk and sour cream and whisk to combine, set aside.

In the bowl of an electric mixer fitted with the paddle attachment, cream the butter for about 1 minute on medium speed. When the butter starts to lighten in color, reduce the speed to low and add the sugar. Increase the speed to medium and allow the butter and sugar mixture to become fluffy. Stop the mixture scrape the sides of the bowl to get it all mixed. Start the mixture and beat in the eggs one at a time waiting until each is incorporated before adding the next. Add the vanilla. Reduce the speed to low and add in 1/3 of the flour mixture, followed by 1/2 of the buttermilk, and continue until you finish the last of the flour mixture. Do not overmix. Using a rubber spatula, give the mixture a final mix.

Spread the cake batter into the prepared sheet pan. Smooth it out into an even layer with your rubber spatula. Bake the cake for 21-25 minutes stopping at the 12 minutes mark and rotating the pan. The cake is done when a toothpick inserted in the center comes out clean. Mine took exactly 22 minutes. Allow the cake to cool completely before frosting. See notes.

FROSTING: In the bowl of an electric mixer fitted with the whisk attachment, whip the butter for 1-2 minutes or until the butter is light and fluffy. Using a rubber spatula, remove the butter from the bowl into a small bowl. Add the cream cheese and whip for 30 seconds or until the cream cheese is light and fluffy. Add the butter back into the cream cheese mixture along with the lemon juice and 3 1/2 cups of the powdered sugar. Start the machine on the stir setting until mostly combined, then increase the speed and whisk for another 1-2 minutes. If your frosting isn't holding its shape, add the remaining 1 cup of confectioners sugar in and whisk to combine.

Spread the frosting all over the cooled cake. Top with sanding sugar and fresh berries if desired. The cake will last covered in the refrigerator for up to 5 days.