Heat oven to 350 degree F. Line a 9 inch by 13 inch non-stick baking sheet with a layer of parchment and then grease the parchment and any exposed pan thoroughly with butter.
Whisk together the dry ingredients (flour, cornstarch, salt, and baking powder) in a large bowl. Set aside.
In the bowl of a standing mixer, cream together the butter and granulated sugar until fully incorporated and very light and fluffy (about 5 minutes).
In a small bowl, beat the egg whites thoroughly with a whisk until they become frothy. This will aerate the egg whites for a lighter batter. Beat the aerated egg whites into the butter-sugar mixture one small tablespoon at a time, making sure the egg is fully incorporated before adding more. The batter should be very light and airy. Beat in the vegetable oil, vanilla extract, and almond milk one at a time.
Turning the mixer to low speed, mix the dry ingredients into the wet ingredients one cup at a time. Remove the bowl from the standing mixer and gently fold the rainbow sprinkles into the batter with a flexible rubber spatula. Be careful not to mix too rigorously, as doing so could break the air bubbles you’ve created with all of your mixing and whisking.
Pour the batter into the prepared baking sheet and spread it into a smooth, even layer. Bake 15-20 minutes, until the sponge cake is light golden in color and a toothpick can be inserted and come out clean. Allow the cake to cool, and then freeze for 30 minutes to overnight.