Preheat oven to 350°.Beat butter and sugars together with an electric mixer in a large mixing bowl 3 minutes at medium speed or until light in color. Set a timer; this step is important. Add eggs, one at a time, beating just until each one is incorporated. Add vanilla and beat at low speed just until incorporated
Combine flour, baking powder, and salt in a medium size bowl. I like to use a large paper plate so I don’t have to wash another bowl. Add flour mixture and toffee bits to mixing bowl; beat just until flour is incorporated.
Line a 13 x 9-inch baking pan with aluminum foil, allowing edges to overhang 2 inches on the end. This will help remove the brownies for easy cutting. Coat the foil with cooking spray or brush lightly with oil. Dollop batter on bottom of lined pan, then spread the batter evenly to the edges of the pan. Bake for 30 minutes or until toothpick inserted in the center comes out almost clean. Don’t over bake them. You want them to be a little soft and gooey. Let cool in pan 20 minutes on a wire rack.
Meanwhile, prepare the icing. Combine brown sugar, evaporated milk, butter, and corn syrup in a medium saucepan. Bring to a boil over medium heat, stirring often. Boil 5 minutes without stirring. Set a timer. Remove from heat and let cool 15 minutes. Stir in powdered sugar and vanilla.
Beat caramel mixture in the saucepan with a handheld electric mixer about 5 minutes or until it starts to thicken. It will still be pourable but will thicken as it cools. Pour icing over brownies in pan, spreading it to the edges. Let the brownies and icing cool completely and icing is firm, about 1 to 2 hours, before cutting.