Preheat the oven to 350°F. Line a 9x9 inch baking pan with parchment paper and set aside.
In a medium bowl, cream together the butter and sugar until light and fluffy. Mix in the egg and vanilla until fully combined. Slowly mix in the flour, baking powder and cornstarch until fully incorporated. Fold in the butterfingers.
Press the mixture into the prepared baking pans, and place in the preheated oven for 30-35 minutes, or until the top looks firm and golden.
Remove from the oven and allow to cool in the pan completely before removing to cut into squares.