To make Pastry: Combine 1/2 cup butter, white sugar, vanilla and the egg in a double boiler over low heat. Stir until thick. Add the graham cracker crumbs, coconut and nuts and mix well. Pour into baking pan and let cool.
To prepare Filling: In medium-sized bowl, dissolve the instant coffee in the hot coffee. Mix in powdered sugar, cocoa, 1/4 cup of butter and milk. Spread filling over pastry and chill for 15 minutes.
To prepare Topping: Melt white chocolate in double boiler over low heat. Spread over filling and cut into squares. Store in refrigerator.