Pre heat oven to 350 degrees. Prepare 9x13 pan or 9x9 for thicker brownies. Line pans with cooking spray or parchment paper.
Combine melted butter and sugar in a large bowl. Stir well.
Add eggs, yolk, one at a time, stir well after each addition.
Stir in vanilla extract
In a separate medium sized bowl, whisk together the flour, cornstarch, baking powder and salt.
Gradually, stir the dry ingredients into the wet ingredients until completely combined. Stir in butterscotch chips.
Spread batter evenly into prepared pan. Bake on 350 degrees until edges just begin to turn golden brown 35-40 minutes.
Allow to cool before cutting.
