Preheat oven to 325ºF. Line four** 8-inch round baking pans with parchment paper and spray with nonstick cooking spray.

In a medium bowl, whisk together flour, baking powder, and salt; set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat butter and shortening together until smooth. Add sugar and beat on med-high until pale and fluffy, about 2-3 minutes.

Reduce speed and add egg whites one at a time, beating well after each addition. Add vanilla and almond extract. Mix in half of the dry ingredients, followed by half of the milk, followed by the remaining dry ingredients, and finally the remaining milk. Try not to overmix. Gently fold in the sprinkles.

Pour batter evenly among the prepared pans, smoothing the tops with a spatula. Bake 30 - 34 minutes, or until a toothpick inserted into the center comes out with few moist crumbs attached. Allow cakes to cool for 10 minutes before transferring to a wire rack to cool completely.

In the bowl of a stand mixer fitted with the paddle attachment, beat the cream cheese until smooth. Add the rainbow chip frosting, beating until combined. Fold in the whipped topping. Refrigerate mousse for about 30 minutes to allow it to firm up a bit before spreading between the layers.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and salt on medium speed until creamy, about 2 minutes. Reduce the speed to the lowest setting and gradually add the powdered sugar, beating well after each addition. Add the heavy cream, vanilla and almond extract. Increase to high speed and beat until very light and fluffy, about 3-4 minutes.

Place one cake layer on a cake pedestal or platter, spread about a third of the mousse on top. Repeat with the remaining cake layers and mousse. Using an offset spatula or knife, frost the sides and top of the cake with the buttercream. You'll have some leftover for the star border. Carefully press sprinkles along the bottom 3 inches of the cake, and then on the top.

Divide the remaining buttercream among 3 or 4 bowls and dye each separately with desired colors. Using a spoon or spatula, spoon buttercream into sections in a pastry bag that's fitted with a large star tip. (See photo above.) You might need to do a few practice swirls on parchment paper before you get all 3 or 4 colors to come out. Pipe stars around the top border of the cake. Refrigerate cake until ready to serve.