Preheat oven to 350°F degrees. Lightly grease a 13 x 9 baking pan and line with parchment paper. Set aside.
In a large bowl, combine melted butter and sugars. Stir well to combine.
1 cup unsalted butter, 1 ⅔ cups brown sugar, ⅔ cup granulated sugar
Stir in eggs, egg yolk and vanilla extract until completely combined.
2 eggs, 1 egg yolk, 1 tablespoon vanilla extract
Add flour, corn starch, baking powder and salt. Stir just until combined.
2 ¼ cups all purpose flour, 2 tablespoons corn starch, 1 teaspoon baking powder, 1 teaspoon salt
Fold in chips of your choice.
1 cup butterscotch, peanut butter, semi-sweet, or white chocolate chips
Transfer batter to the prepared baking dish and spread evenly.
Bake for 22 to 27 minutes or until an inserted toothpick near the edge comes out with just a few moist crumbs.
Let cool completely before cutting into bars and serving.