Place the bottom cake round on a cake board the same size as your cake.
Frost and top with the second layer.
Cut holes in the centers of the middle two tiers using a round cookie cutter. Frost between the two tiers up to the holes.
Fill the hole with sprinkles.
Place the next two layers on top of the tier to cover up the hole, frosting between them.
Crumb coat and lightly ice your cake. It does not have to be perfect! You will be covering the entire cake with sprinkles so imperfections are totally okay.
After the cake is iced, chill it completely until the icing is firm when you touch it. The cake must be cold for the next step.
Prepare a sheet pan or jelly roll pan with parchment paper on the bottom. Pour sprinkles on the parchment paper, covering the entire surface.
Place another cake board the same size as the cake on top of the top layer. Carefully lift up the cake, place it on its side and roll into the sprinkles.
Once the sides are coated, take off the cake board from the top of the cake and cover the top of the cake with sprinkles.
When you cut into the cake, you'll get a surprise of sprinkles pouring out!
Video
Notes
The time estimate does not include baking or chilling the cakes.