Beat 3/4 cup butter at medium speed of an electric mixer until creamy; gradually add brown sugar, beating well. Add egg yolk, beating well. Add flour, coffee granules, and salt, stirring just until blended. Using greased fingers, press dough into bottom of a lightly greased 13" x 9" x 2" pan. Bake at 350° for 20 minutes. Set aside.
Combine sweetened condensed milk and 2 tablespoons butter in a saucepan. Bring to a boil over medium heat; cook 5 minutes, stirring often. Remove from heat, and stir in Kahlúa. Pour mixture over prepared crust. Sprinkle with walnuts.
Bake at 350° for 10 to 12 minutes or until golden. Remove from oven. Sprinkle with chopped coffee beans and toffee bars while still warm. Cool completely in pan on a wire rack. Cover and chill until firm. Let stand 5 minutes before cutting into bars.