In a large saucepan melt the butter and stir in the cocoa and sugar. Add 1 beaten egg with the vanilla to the melted butter. Add the 2 cups of "Graham" wafers and mix well. Press this mixture into the bottom of a lightly greased 9*9 inch pan. Place in the refrigerator. FILLING: Beat together icing sugar, butter, and custard. Add a little milk if needed to make a smooth dough. Spread on first layer and return to the refrigerator. TOPPING: Melt chocolate, butter and vanilla in a saucepan. Work quickly to get a smooth even layer. Return to the refrigerator to set. Cut into bars. Refrigerate.