Combine 1/2 cup of butter and cocoa in saucepan over low heat and stir until
blended. Remove from heat and add 1/2 cup confectioners' sugar, egg and
vanilla extract. Stir in Graham cracker crumbs. Mix well and press mixture into
the bottom of a 13 x 9-inch pan.
Melt 1/2 cup butter and pour into a small mixing bowl. Add creme de menthe
and mix at low speed. While mixing, add remaining 3 cups confectioners'
sugar until smooth. Spread this mixture over the first layer in the 13 x 9-inch pan.
Refrigerate entire dish for one hour.
While the dish is chilling, combine 1/4 cup butter and chocolate pieces
in a saucepan over low heat. Stir until melted. Remove pan from refrigerator
and spread chocolate mixture over the mint layer.
Return pan to the refrigerator for 2 hours.
Cut into small squares and store in refrigerator until ready to serve.