Preheat the oven to 350 degrees F. Adjust the racks to the center of the oven. Spray an 8-by-8-inch baking pan with nonstick spray.
Melt the butter in a medium saucepan and stir in the brown sugar until melted together. Let cool for 5 minutes.
While the mixture is cooling, whisk together the flour, baking powder and salt in a medium bowl.
Toast the pecans in medium skillet over medium heat. Remove to a plate once toasted.
Stir in the eggs, one at a time, and vanilla extract into the cooled sugar and butter mixture. Add the dry ingredients into the saucepan until combined. Fold in the toffee and chopped pecans; the batter will be very thick.
Spread the mixture into the prepared pan. Bake until golden brown and a toothpick inserted into the center comes out clean, about 45 minutes. Cool for 15 minutes before slicing into squares.