1. Using a serrated knife, trim domes from tops of cakes. Transfer one cake layer to serving platter, and spread with about 1 cup frosting. Top with second cake layer. Use remaining frosting to cover cake, spreading with an offset spatula.
2. To make the candy numbers, lightly spray a piece of parchment paper and metal cookie cutters with nonstick cooking spray. In a microwave-safe glass measuring cup with a pour spout, heat hard candies on high until melted and almost boiling, about 2 minutes, stopping and swirling every 45 seconds to prevent burning (do not stir). Carefully pour melted candies into prepared cookie-cutter molds. (Warning: Candy will be very hot!) Let stand at room temperature until cool, about 20 minutes. Gently remove candy from mold and place in the center of the cake.
3. Arrange candies over top and sides of cake, pressing to adhere to frosting.