Preheat oven to 350 degrees F. Lightly spray pan with nonstick cooking spray.
In a small bowl, whisk together the flour, baking powder, and salt.
In another bowl, cream together the butter, brown sugar, and granulated sugar until light and fluffy. Add the eggs, one at a time, beating well after each addition.
Blend in the vanilla, and then the flour mixture; do not overmix. Gently fold in the 3/4 cup toffee bits.
Spread batter evenly in the prepared pan. Sprinkle with the remaining toffee pieces.
Bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean. Cool in pan on a wire rack before cutting.