Stir together butter, chocolate, and coffee in a heavy saucepan. Cook, stirring occasionally, over low heat until butter melts; cool slightly.
Beat eggs at high speed with an electric mixer. Gradually add sugar and vanilla, beating 3 minutes or until thick and pale. Add chocolate mixture, beating until blended. Gradually add flour, beating at low speed just until blended. Stir in chocolate morsels and walnuts. Pour into a 13- x 9-inch pan lined with aluminum foil.
Bake at 375° for 35 minutes or until set.
Cool in pan on a wire rack. Lift foil with brownies out of pan. Cut brownies into squares.