Prep. Preheat oven to 325°F. Spray a 12-cup bundt pan with nonstick baking spray.
Combine. In the bowl of a stand-up mixer, set on medium speed, beat butter (1 1/2 cups), brown sugar (2 cups), and granulated sugar (1 cup) for 4 minutes or until light and fluffy. Scrape down the sides of the bowl. Add the eggs, 1 at a time, until the yolks disappear.
Dry ingredients. Add the flour (3 cups), baking powder (1 teaspoon), and salt (1/2 teaspoon) to a medium mixing bowl. Whisk until combined. Reduce the mixer speed to low and add the flour mixture alternatingly with the milk, beginning and ending with the flour mixture mixing just until combined after each addition.
Fold in toffee bits (8 ounces) and pecans (1 cup) until evenly combined. Pour into prepared pan.
Bake in preheated oven until a skewer inserted in the center of the cake comes out clean, about 1 hour and 15 minutes to 1 hour and 25 minutes. Check during the baking time for excess browning and cover with foil if necessary. Let cool in pan set on a wire rack for 15 minutes. Invert pan and place cake on serving plate or cake stand. Allow the cake to cool completely before beginning the glaze.
Icing. Bring condensed milk (14 ounces) and brown sugar (1 cup) to a boil over medium-high heat, whisking almost constantly. Reduce the heat and simmer for 8 minutes, whisking often. Take off of the heat and stir in butter (2 tablespoons) and vanilla (1/2 teaspoon).
Assemble. Cool for 5 minutes before drizzling on the cake. Allow glaze to harden before slicing, and enjoy!