If desired, sift powdered sugar over brownie squares before serving.
Soak chopped walnuts in liqueur 4 to 6 hours. Drain, discarding liqueur. Set walnuts aside.
Melt butter and chocolate in a heavy saucepan over low heat.
Beat eggs, sugar, and coffee mix at medium-high speed with an electric mixer 8 minutes. Gradually add chocolate mixture, beating at low speed until blended. Gradually add vanilla, flour, and salt, beating until blended. Stir in walnuts. Pour into a lightly greased aluminum foil-lined 13- x 9-inch pan.
Bake at 350° for 35 to 45 minutes. Cool completely on a wire rack. Cut into squares.
Note: For testing purposes only, we used International Coffees Suisse Mocha for coffee mix.