Combine butter and sugars, mix on medium for around 2 minutes, until light and fluffy.
Cream Butter & Sugars
Stir in vanilla and egg, making sure to mix until fully combined.
Add Egg and Vanilla
Scrape down the sides of the bowl again. You don't want any trace of egg left behind.
Scrape the bowl
Gently fold in the flour, baking soda, salt, cornstarch, and chocolate chunks
Add the rest of cookie ingredients
Fold until just combined, over-mixing can create at tough cookie.
Use the easy one-pot brownie recipe provided on the website or use your favorite box mix.
Make Brownie Batter
Pour brownie batter into a prepared pan. Take balls of cookie dough & flatten into a sheet and place on top of brownie batter until all cookie dough is used up.
You want a toothpick in the center to come out with moist crumbs, not completely dry. Place the pan on a wire rack to cool completely before cutting.
Bake and Cool