Fill plastic bag with some chocolate frosting. Seal bag and cut a very small end off one corner of the bag and set aside. For each hat, invert one ice cream cone and fill with about 2 tablespoons candy corn or small candies. Pipe some frosting from bag along bottom edge of cone. Press a cookie against frosting. Carefully invert right side up onto parchment or waxed paper-lined cookie sheet.
Decorate outside of cone with small candies, using additional frosting as necessary. Refill frosting bag as needed. If desired, roll out large green and yellow gumdrops on a sugared surface until 1/8 inch thick. Cut into 1/4-inch strips. Press green strips around brim of hat and cut smaller pieces from yellow strips to make a buckle; press onto hat. Makes 20 hats.