1 1/2 sticks (12 tablespoons) unsalted butter
2 1/4 cups packed light brown sugar
2 cups all-purpose flour
2 teaspoons baking powder
Pinch of salt
3/4 cup pecans, chopped
2 large eggs
2 teaspoons vanilla extract
1 cup toffee pieces
Preheat the oven to 350 degrees F. Spray an 8-inch-square baking dish with cooking spray and line the bottom and sides with parchment paper. Spray the parchment with cooking spray.
Melt the butter in a medium saucepan over medium heat; add the brown sugar and stir until melted. Let cool 5 minutes. Meanwhile, whisk the flour, baking powder and salt in a medium bowl. Toast the pecans in a medium skillet over medium heat, 6 to 8 minutes; remove to a plate. Stir the eggs, one at a time, into the cooled butter-sugar mixture, then stir in the vanilla. Add the flour mixture and stir until combined. Fold in the toffee and toasted pecans (the batter will be very thick).
Spread the batter in the prepared baking dish. Bake until golden brown and a toothpick inserted into the center comes out clean, about 45 minutes. Let cool 15 minutes, lift out of the pan and cut into pieces.