Prepare cake according to package directions in two greased and floured 9 inch cake pans. Cool completely and cut both of the layers in half horizontally to make four layers total. Place one layer on a serving plate. In a small saucepan, melt chocolate and butter. Stir till smooth. Stir in walnuts. It will be thin consistency. Spread 1/2 of the chocolate on the 1st cake layer. Top with another layer of cake. Spread half the cool whip on top. Top with another cake layer. Spread the remaining chocolate on this layer and top with last cake layer. Spread with remaining cool whip.