Preheat oven to 350 degrees F. Lightly grease a 15x10x1-inch baking pan or line the pan with foil or parchment; set aside. For cookie base, in large mixing bowl beat the 3/4 cup butter on medium to high for 30 seconds. Add brown sugar; beat until combined, occasionally scraping sides of bowl. Beat in egg, vanilla, and salt until combined. Beat in as much flour as you can with mixer. Stir in any remaining flour. Evenly spread dough in prepared pan.
Bake for 20 to 22 minutes until edges are golden brown. Cool completely on wire rack. Makes 36 bars.
For frosting, in small saucepan combine chocolate and 3 tablespoons butter. Stir over low heat until melted and smooth. Remove from heat. Transfer chocolate mixture to a medium bowl. Whisk in coffee and peanut butter until combined. Whisk in powdered sugar until smooth. Spread on cookie base. Let stand until frosting is set.