1Heat oven to 350°F. Spray 13x9-inch pan with cooking spray. Make brownie batter as directed on box, using water, oil, eggs and stirring in toffee bits. Pour into pan. Bake 45 to 55 minutes. Place pan on cooling rack.
2When brownies come out of oven, in 2-quart saucepan, melt butter over medium heat. Stir in cocoa, salt, vanilla and whiskey. Heat to boiling, stirring constantly. Slowly, beat in powdered sugar with whisk. Cook 5 minutes longer.
3With bamboo skewer or knife, poke holes in brownies. Pour hot frosting over warm brownies; spread over top until smooth. Cool at least 2 hours before cutting. For bars, cut into 8 rows by 4 rows.