Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans.

Whisk together the all-purpose flour, cornstarch, baking powder, and salt in a medium bowl. Set aside.

Cream the vegan butter and sugar together in a large bowl until light and fluffy.

Add the egg whites one at a time, beating well after each addition.

In a separate bowl, combine the oat milk and vanilla extract.

Gradually add the oat milk mixture to the butter and sugar mixture, alternating with the flour mixture. Begin and end with the flour mixture, mixing until just combined.

Fold in the rainbow sprinkles gently until evenly distributed throughout the batter.

Divide the batter evenly among the prepared cake pans.

Bake in the preheated oven for 35 to 40 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cakes to cool in the pans for 10 minutes before transferring them to wire racks to cool completely.

While the cakes are cooling, prepare the frosting. In a large bowl, beat together the vegan butter, vegetable shortening, and vanilla extract until creamy.

Gradually add the powdered sugar, one cup at a time, beating well after each addition.

Add oat milk as needed to reach your desired consistency. Beat until smooth.

Once the cakes are completely cool, assemble the cake by spreading a layer of frosting between each cake layer and covering the top and sides with the remaining frosting.

Decorate the cake with additional rainbow sprinkles, if desired.