Time to stack! Cut off the convex top of the cakes (easiest using a cake saw), so that we can make a stable pile. If necessary, also cut the harder, brown edges of the cake.
Grab your frosting. Place the bottom layer (purple) on a flat plate and spread a generous layer of frosting on it. Place the next layer (blue) on the frosting layer. Press this briefly, so that the cake is straight. Spread the blue cake with frosting. Repeat this with all layers.
If your last layer (black or red) is on the stack, press the cake well again. The frosting will come out a little on the sides, but that's good! Spread this on the sides. Spread an extra layer (not too thick) over the sides and top.
Put the cake in the fridge for at least 30 minutes, so the buttercream hardens and your cake becomes firmer.
Spread another layer of frosting on the side and top, so that you do not see any of the cakes anymore. You get the best result with thin layers (with a cooling period in between). Smooth the sides with a (glazing) knife.
Fill a piping bag with the rest of the frosting and decorate the top of the cake with pretty swirls.
Decorate the cake with sprinkles. We sprinkled the top and spooned sprinkled onto the bottom of the cake.
Keep the cake in the refrigerator until you eat it. Enjoy!