Preheat the oven to 350°F. Spritz a metal non stick 9 x 13 inch pan with baking spray. Set aside.
On the stove top, melt together the butter and brown sugar. Cook over low heat for 2 minutes until the sugar is dissolved, then remove to a mixing bowl to cool.
After 5-10 minutes, beat the eggs into the brown sugar-butter mixture, one at a time.
Add the vanilla. If the mixture is just warm to the touch, it's fine to add the eggs.
Sift together the flour, baking powder and salt. Add to the wet ingredients.
Beat with a hand mixer for 2-3 minutes until fully combined.
Mix in 1 cup of crushed Butterfingers by hand.
Pour into the prepared pan, and sprinkle the remaining 1/2 cup of crushed Butterfingers on top.
Bake for 25 minutes. Test the center with a toothpick before removing from the oven. Look for moist crumbs, not a clean toothpick. Do not over bake.
Cool to room temperature in the pan on a wire rack before cutting.