Preheat the oven to 350°F and line an 11x15-inch sheet pan with parchment paper (or lightly grease and flour).
In a large bowl, combine the melted butter and brown sugar. Stir well.
Add peanut butter and mix until creamy.
Add the eggs and the egg yolk, one at a time, stirring well after each addition.
Stir in the vanilla extract.
In a separate, medium-sized bowl, whisk together the flour, cornstarch, baking powder, and salt.
Gradually stir the flour mixture into the butter and egg mixture until completely combined.
Spread the batter evenly into the prepared pan. Firmly press the batter into the pan into a flat, even layer. (We used another piece of parchment to press down on the batter.)
Bake for 15-20 minutes, or until the edges just begin to turn golden brown.
Allow the bars to cool completely before adding the peanut butter sauce.