Preheat oven to 350°F (175°C) and prepare a 9x9-inch pan by lining it with parchment paper (or lightly grease and flour). (You could use a 9x13-inch pan for more servings, but the bars will be thinner.)

Combine melted butter and brown sugar in a large bowl. Stir well.

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. Stir in butterscotch chips.

Spread the batter evenly into the prepared pan. Bake at 350°F (177°C) until edges just begin to turn golden brown (33-36 minutes). If you are using a 9x13-inch pan, bake for 25-30 minutes.

Allow to cool before cutting and serving.