Line two greased 15x10x1-in. baking pans with waxed paper. Prepare cake mix according to package directions. Divide batter evenly into prepared pans, spreading evenly.
Bake at 350° for 12-15 minutes or until tops spring back when lightly touched. Cool for 5 minutes.
Invert each cake onto a kitchen towel dusted with confectioners' sugar. Gently peel off paper. Roll up cake in the towel jelly-roll style, starting with a short side. Cool completely on a wire rack.
In a small bowl, mix whipped topping and extract until blended. Unroll cakes; spread each with 1-1/2 cups peppermint mixture to within 1/2 in. of edges.
Cut a small hole in the corner of a food-safe plastic bag; fill with fudge topping. Drizzle each cake with half of the fudge topping; sprinkle with 2 tablespoons crushed candies. Roll up again, without towel.
Transfer to platters. Frost with remaining peppermint mixture. Cakes can be served immediately or frozen, covered, for later use. Remove from the freezer 10 minutes before serving. Top with remaining candies and, if desired, mint leaves just before serving. Yield: 2 cakes (10 slices each).