1. Heat oven to 350 degrees F. Line two baking sheets with parchment paper. In a large mixing bowl, beat together one 16.5-oz. pkg. white cake mix, 1/3 cup melted unsalted butter, 2 egg whites, 2 Tbs. water, and several drops of purple food coloring until combined. Using a cookie scoop, portion batter into 24 cookies (about 2 Tbs. each) 2 inches apart on prepared baking sheets. Bake until puffy and set, about 10 minutes. Transfer cookies to a cooling rack. Cool completely.
2. Tint a 16-ounce can of white frosting with neon green food coloring. Spread the bottom of a cookie with 2 Tbs. frosting and sandwich with another cookie. Repeat with cookies and frosting. Use additional frosting to attach candy eyes and small white oval mints (such as TicTacs) for teeth. If desired, add a snipped piece of bubble gum tape for a tongue. Makes 12 servings.