1. Preheat oven according to box directions. Grease two, round 8-inch cake pans with baking spray and set aside. In a large bowl pour the cake mix, butter, mayonnaise, eggs, and water, and using a hand mixer blend until well combined. Bake according to the box.
2. Once baked, let cakes cool for 10 minutes in the pan, then carefully turn out on a cooling rack to cool completely. Meanwhile, make the buttercream frosting.
3. Once cakes are cooled, spread a generous layer of frosting on the flat side of one of the cakes. Place the other cake on top, making sure both flat sides are touching each other (with frosting in between). Frost the rest of the cake until fully covered and top with rainbow sprinkles! This cake is even better the next day!

For the chocolate buttercream

1. Add cocoa to a large bowl. Whisk to remove any lumps. Cream together butter and cocoa powder until well combined.
2. Add sugar and milk to cocoa mixture by adding 1 cup of sugar followed by about a tablespoon of milk. Each time after adding a little sugar and milk, beat on high for about 1 minute. Repeat this process until all the sugar and milk has been added. Add vanilla extract and blend.
3. If the buttercream seems too dry, add more milk, a tablespoon at a time until it reaches the right consistency. If it appears to wet, add more powdered sugar, a tablespoon at a time until it reaches the right consistency.