Mise en place. Preheat your oven to 350˚F (175˚C). Grease and flour two 8-inch cake pans. Then, gather your ingredients and measure them out before starting.
Make the batter. In a mixing bowl, whisk the flour, cornstarch, baking powder and salt together until combined. Set the bowl aside for later.
Add the butter to the bowl of an electric mixer and beat until light and fluffy. Then, add the sugar and continue to beat until fully creamed and lighter in color - about 1 minute. Next, add the egg whites, one at a time, and beating to incorporate between each addition.
Stir the vanilla extract into the dairy free milk. Next, alternate between adding some of the milk mixture and some of the flour mixture, beating each addition until it’s incorporated. Scrape the sides as needed and continue to beat until the batter is smooth. Then, gently stir in the sprinkles with a rubber spatula.
Bake the cakes. Divide the batter between your prepared pans and bake the cake layers for about 35 - 45 minutes. The cake is done when the center no longer wobbles and an inserted toothpick comes out clean. Remove them from the oven and allow them to cool for about 10 minutes before carefully flipping them onto a cooling rack.
Make the frosting. With an electric mixer, beat the dairy free butter and shortening until combined. Then add 1 tablespoon of milk, the vanilla bean paste, a pinch of salt (if using). Then, mix in powdered sugar - in 3/4 cup increments - until incorporated. As you add sugar, you will need to add more dairy free milk to help mix and to get to your desired thickness. I generally add about 3 - 4 tablespoons total.
Assemble your cake. When the cake has completely cooled, spread a thin layer of frosting on top of one layer, then add the next layer. Use the rest of the frosting to cover the cake. If you have extra frosting, it can be used for piping or discarded.