Preheat oven to 350 degrees F. Line a 9-inch square baking pan with foil, extending the foil up and over the edges of the pan. Grease foil; set aside.Advertisement
Prepare brownie mix according to package directions, except stir the 1/2 cup toffee pieces and the espresso powder into batter. Spread batter into prepared pan.
Bake in preheated oven for 35 minutes. Cool in pan on a wire rack. Remove baked mixture from pan by lifting foil. In a small bowl, combine vanilla frosting and liqueur. Spread frosting mixture onto cooled brownies. Sprinkle with the 1/4 cup toffee pieces. Cut into bars. Makes 20 brownies.
Place bars in a single layer in an airtight container; cover. Store in the refrigerator for up to 3 days.