Once cakes have cooled, add frosting ingredients to a large bowl or stand mixer fitted with a whisk attachment and beat until light and fluffy, about 3 to 4 minutes, scraping down the sides of the bowl as needed.
Place bottom cake layer on a plate or cake stand and add strips of wax paper under it on each side about a 1/2 inch in under the cake, this will allow you to pull it away later and have a clean stand/plate when you're done frosting.
Dollop some frosting on the top and work it evenly over the cake with a frosting spatula, add the next layer and do the same, continue if you're doing a 4 layer cake.
Use the frosting spatula to add a thin crumb coat around the edge of the cake, then work from the top down the sides of the cake to do the final frosting layer. The sides don't have to be perfect because we'll be covering it up with sprinkles.
Add sprinkles to the sides of the cake by holding sprinkles in your palm and gently pressing them against the side.
Add some of the frosting to a piping bag or decorator tool with a large star tip and pipe along the edge of the top of the cake. Add a Sixlet to the top of each peak.
Add more sprinkles in a circle in the top middle of the cake. Gently pull the strips of wax paper out from under the cake.
Serve as soon as possible or store in the fridge for up to two days. However, refrigerating will dry out the cake.