Trim the top and bottom off the cakes with a large, serrated knife or cake leveler to remove the crust and create flat, even layers.
Attach an 8-inch cake board onto a 10-inch cake board with a little buttercream. Place onto a cake turntable and attach one cake layer onto it with a little buttercream. Pipe a thin layer of frosting over the cake layer, smooth with a cake scraper if necessary, then add the second layer on top. Repeat with more frosting and add the third layer.
Trim the sides of the assembled cake with a serrated knife to create a flat surface.
Use an offset spatula to apply a thin crumb coat of buttercream. Chill the cake for 15-20 minutes until set.
Add more buttercream to the top and sides of the cake and use a cake scraper to create a smooth top and sides. Chilling the cake at intervals makes this process easier.
Press sprinkles into the side of the cake around the bottom edge going up around halfway. Running a cake smoother around the base of the cake over the sprinkles helps to flatten them into the sides of the cake. Chill until firm.
Place a large, closed star nozzle into a large piping bag and fill with the reserved frosting.
Pipe 12 rosettes around the top edge of the cake.
Sprinkle a few sprinkles onto each rosette.
Transfer the cake to a cake stand to serve.