Chocolate Cafe au Lait Cake

1/2 cup (1 stick) butter or margarine
1 1/2 cups graham cracker crumbs
6 ounces semisweet chocolate chips
1/2 cup chopped walnuts
1 (18.25 ounce) box devil's food or chocolate fudge cake mix
1 1/2 cups heavy cream
1 1/2 teaspoons instant coffee powder
1/3 cup sifted confectioners' sugar

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 pans. 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. Refrigerate until serving time.