Bake brownies according to manufacturer’s instructions. Let cool completely, cut into bars (we cut them into 2.5 inch by 3 inch bars, but you can cut whatever size works best for your shop). Slightly warm icing and spread over brownies, being careful to spread as evenly as possible. Melt white coating chocolate and load into a very small piping bag. Pipe the letters “A, B, C” on top of each brownie. Decorate with a sugar apple if desired.