Yields one 3-layer 8-inch cake or 24 cupcakes.
Preheat the oven to 350ºF. Grease the bottoms of three 8-inch cake pans with butter and line with parchment or line two 12-cup cupcake tins with liners and set aside.
Whisk together the flour, cornstarch, baking powder and salt in a medium bowl.
In the bowl of stand mixer fitted with a paddle attachment, cream together the granulated sugar and butter until light and fluffy, 3 to 4 minutes. Add the egg whites, one at a time, mixing well after each. Add the oil and the extracts.
With the mixer running on low, add the dry mixture and the milk in 2 or 3 alternating batches and mix until just barely combined. Gently fold in the sprinkles using a rubber spatula until they’re evenly distributed. Distribute the batter among the cake or cupcake pans, spreading it out evenly if using cake pans, and bake until lightly browned on top and a toothpick inserted into the center comes out clean. Begin checking for doneness at 25 minutes for cakes and 20 minutes for cupcakes. Let cool in the pans for 10 minutes and then turn out onto a wire rack to cool completely.
Beat the butter until smooth with the whisk attachment and gradually add the confectioners’ sugar until creamed. Add the milk, extracts and salt and beat to combine. Decorate as desired and enjoy.