Golden Sheet Cake with Berry Buttercream

12 tablespoons (170 grams or 1½ sticks) unsalted butter, at room temperature, plus more for pan
2¾ cups (345 grams) all-purpose flour
¼ cup (30 grams) cornstarch
1¼ teaspoons baking powder
½ teaspoon baking soda
¾ teaspoon table salt
1½ cups (300 grams) sugar
1½ teaspoons vanilla extract
3 large eggs, at room temperature
1½ cups (355 grams) buttermilk, well shaken

After all, nature provides us with such a pretty palette of berries—surely they can brighten any cake.

Preheat your oven to 350 degrees. Butter a 9-by-13-inch cake pan, and line the bottom with parchment paper. Coat the parchment paper and the inner sides of the pan with butter or a nonstick cooking spray.
Whisk together the flour, cornstarch, baking powder, baking soda, and salt in a medium bowl. In a large mixing bowl, beat the butter and sugar with an electric mixer at medium speed until the mixture is pale and fluffy, then beat in the vanilla. Add the eggs one at a time, beating well and scraping down the bowl after each addition. At low speed, beat in the buttermilk until just combined (the mixture will look curdled). Add the flour mixture in three batches, blending until each addition is just incorporated into the batter.
Spread the batter evenly in the prepared cake pan. Bake until it is golden and a wooden pick inserted in center of cake comes out clean, 30 to 40 minutes. Cool the cake in the pan on a rack 10 minutes, then run a knife around edge of pan. Invert the cake onto rack and discard the parchment. Cool completely, for about 1 hour. Alternatively, you can leave the cake inside the pan and only frost the top, as is more traditional for sheet cakes (plus, the cake pan becomes your cake carrier).