Slice the tops and bottoms off of each cake, to create flat cakes and remove any excess browning. Place a cake board on top of each cake and trim off the browned edges, so that the cakes are the same size as the cake board.
Slice each cake in half to create 2 layers per cake.
Place the cake board onto your work surface. Stack the four layers of cake on top, spreading some buttercream evenly between each layer.
Coat the cake evenly in buttercream. Place a second cake board on top of the cake.
Line a cookie sheet with parchment paper and fill with the remaining 3 cups confetti sprinkles. Turn the cake on its side, with one hand on each cake board, and roll the sides of the cake in the sprinkles, until all sides are evenly covered. Return the cake to your work surface.
Remove the top cake board and smooth the buttercream, if it has shifted.
Sprinkle the top with the remaining sprinkles. Enjoy!