In a bowl, mix melted butter, granulated sugar, brown sugar, egg yolk, and vanilla until smooth.
Stir in baking soda and salt.
Using a spatula, mix in flour just until no dry flour is showing.
Add most of the chocolate chips, saving a few for the top.
Prepare brownie batter according to the package directions and spread evenly in a 9×9-inch pan. Spoon dollops of cookie dough on top, gently spread with a knife, then sprinkle remaining chocolate chips.
Bake according to the package directions. If the cookie layer browns too fast, loosely cover with foil.
Let bars chill in the fridge for at least an hour before cutting.
Video
Notes
Brownie mix: Use an 18-ounce box of any brand.
Water: Substitute the same amount of milk, buttermilk, or cooled coffee.
Instant coffee: Optional. Skip if using brewed coffee.
Butter: Salted or unsalted works.
Egg yolk: Use only the yolk for the cookie dough.
Chocolate chips: Semisweet or bittersweet works. Save a few for topping.