(Nutrition information is calculated using an ingredient database and should be considered an estimate.)
In the bowl of a stand mixer or a large mixing bowl with an electric mixer, cream the butter until smooth. With the mixer on low, slowly add the powdered sugar, mixing until combined.
Add the vanilla and a tablespoon of oat milk at a time, mixing until smooth and spreadable. Fold in a few drops of gel food coloring, as desired. If using multiple colors, divide the icing into small batches.
Core the apple slices with a 1-inch round cookie or biscuit cutter (or with a paring knife).
Before spreading the frosting on, dab each apple slice with a paper towel to dry it well. Spread on a tablespoon of buttercream. Top with sprinkles. Enjoy! Best served same day (see note for preparing in advance).
Invite your kids in to add the sprinkles!
Buttercream is best used soon after you make it, as the powdered sugar will start to firm up as it sits.
If preparing donuts a few hours in advance, you’ll want to keep the apples from browning. Prepare a water bath with 1 cup water and the juice of a lemon. Dunk each apple slice in the lemon bath and dry it well with a paper towel before frosting.