Be sure to pour your sprinkles slowly — you don’t want to overfill the cookie cutter, since they could spill out and ruin the clean edges once the cutter is pulled away. You should only be pouring in enough to cover the buttercream frosting. Then, to make sure the sprinkles stick, you can gently press them down using your finger or a spoon. Once they’re secure in the frosting, you can carefully take the cookie cutter out, leaving behind the sprinkles in the shape of the cutter.
This trick will work with any shape of cookie cutter, so long as sprinkles can be poured in from the top. If you’re celebrating a birthday, you can buy number-shaped cookie cutters to add an age amongst the candles. The sprinkles will add a pop of color to stand out against the frosting, as well as some texture to the top of the cake. This method works with anything sprinkle-like: rainbow jimmies, crushed-up cookie crumbs, chopped nuts, or crushed peppermint.