Start by preheating your oven to 350°F (175°C).
Line your 9x13 inch baking pan with parchment paper.
In a large mixing bowl, combine the melted butter, packed brown sugar, and granulated sugar. Whisk until well blended. Add in the eggs, egg yolk, and vanilla extract. Mix until smooth and creamy.
In another bowl, whisk together the all-purpose flour, corn starch, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, using a spatula to fold them together.
Fold in your choice of chocolate chips.
Pour the thick batter into the prepared baking pan and spread it evenly.
Bake in the preheated oven for about 25-30 minutes, until the edges are golden brown.
Once baked, cool in the pan for about 10 minutes, then lift out onto a wire rack to cool completely before cutting into squares.
