Soak chopped walnuts in liqueur 4 to 6 hours. Drain, discarding liqueur.
Melt butter and chocolate in a heavy saucepan over low heat.
Beat eggs, sugar, and suisse mocha 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 chopped walnuts. Pour into a lightly greased aluminum foil-lined 13- x 9-inch pan.
Bake at 350° for 30 to 35 minutes. Cool on a wire rack. Cut into squares. Garnish, if desired.
NOTE: For testing purposes only, we used International Coffees Suisse Mocha.