Melt butter in a medium saucepan over low heat, or in a 1-quart glass bowl
in a microwave oven on HIGH for 1 to 1 1/4 minutes.
Stir in Graham cracker crumbs and let cool.
When cool, stir in chocolate chips and walnuts; set aside.
Heat oven to 350 degrees F.
Grease and flour two 9-inch round cake pans.
Prepare cake mix according to package directions. Turn batter into prepared
Sprinkle crumb-walnut-chocolate chip mixture over cake batter, dividing
evenly between pans. Using fingertips or back of a spoon, lightly press mixture
into batter (crumb mixture should not sink all the way into batter.)
Bake cake as package directs (30 to 35 minutes), or until cake tester inserted
in center comes out clean.
Remove cakes from oven; let cool completely in pans before unmolding. Keep
layers crumb side up.
In a medium bowl, beat cream, coffee powder and confectioners' sugar
with an electric mixer on high speed until stiff.
Place one cooled cake layer, crumb side up, on a serving plate.
Spread three quarters of whipped cream mixture over cake.
Top with remaining cake layer, crumb side up.
Frost sides of cake with remaining whipped cream; do not frost top of cake.