Melt 1/2 cup butter in a saucepan over low heat; whisk white sugar and chocolate drink mix into butter until smooth. Remove saucepan from heat; whisk in 1 teaspoon vanilla and beaten egg. Stir crushed crackers and coconut into chocolate mixture. Press mixture into the bottom of prepared pan. Refrigerate until set, about 30 minutes.
Beat 3 tablespoons butter and confectioners' sugar with an electric mixer in a bowl until smooth; stir in milk and remaining 1/2 teaspoon vanilla. Spread icing mixture evenly over chocolate-cracker mixture; refrigerate until set, about 30 minutes.
Melt chocolate chips with margarine in a saucepan over low heat; whisk until smooth. Pour chocolate mixture over icing layer; smooth with a spatula. Refrigerate bars until set, about 1 hour. Serve cold or at room temperature.
Use an 8-inch square pan for squares or a 9-inch pan for thinner bars.
This recipe contains raw eggs. We recommend that pregnant women, young children, the elderly, and the infirm do not consume raw eggs. Learn more about egg safety from our article, Making Your Eggs Safe.