Preheat the oven to 350 degrees F. Coat three 9-inch-roundcake pans with cooking spray. Prepare the cake mixes as directed; divide the batter evenly among the pans and bake 30 to 35 minutes. Let cool.
Make the "potato skin" frosting: Beat the butter and milk with a mixer until smooth. Beat in the cocoa powder. Beat in the confectioners' sugar, 1 cup at a time, until the mixture is as thick as cookie dough. Form into 2 disks, wrap in plastic and refrigerate.
Level the domed tops of 2 cakes with a serrated knife. Stack the flat cakes on a cardboard circle with vanilla frosting between each; use the remaining domed layer on top. Freeze 30 minutes.
Trim the cake into a potato shape with a small serrated knife; reserve the trimmings. Cover the cake with a thin layer of vanilla frosting.
Dust a sheet of parchment paper with cocoa powder. Remove 1 disk of chocolate frosting from the refrigerator and roll into a 5-by-10-inch oval, dusting with more cocoa. Repeat with the other disk of frosting.
Use the parchment to press the frosting sheets against the cake as shown, leaving a few inches on top uncovered. Dust with more cocoa. (Don't worry about how the bottom of the cake looks-it will be covered with foil.)
Gently fold back the frosting on top. Crumble about 1/2 cup of the cake trimmings (use the light inside pieces) and sprinkle them on top of the cake.
Unwrap the candies and microwave until soft but not melted, about 12 seconds. Stack the candies, then flatten with your fingers. Set on top of the cake.
When ready to serve, snip the mint leaves on top of the cake. Carefully wrap the bottom half of the cake with a large sheet of foil.