• Line an 8"x8" metal baking pan with parchment paper. Set the pan aside.

Make the brownie batter.

• In a large, microwave-safe bowl, melt butter and chocolate in the microwave in 30 second increments. Whisk until smooth.
• Add both sugars and whisk until combined.
• Add vanilla and egg and whisk until combined.
• Add flour, cocoa powder, and salt and fold into the sugar mixture using a wooden spoon or rubber spatula.
• Preheat the oven to 350°F and set the brownie batter aside while you make the cookie dough.

Make the cookie dough.

• In a large bowl, add melted butter, both sugars, and vanilla and whisk to combine.
• Add egg and whisk well until the mixture is glossy and a bit lighter in color, about 1 minute. Let the mixture stand for about 5 minutes while you prepare the next ingredients.
• In a medium bowl, add flour, cornstarch, baking soda, and salt and whisk to combine.
• Give the sugar mixture a quick whisk and then add the flour mixture. Use a wooden spoon or a spatula to gently stir the flour mixture into the sugar mixture. Stir just until mixed.
• Add both chocolate chips and stir just until blended.

Bake the brookies.

• Use a small cookie scoop or spoon to alternately add scoops of brownie batter and cookie dough to the prepared baking pan.
• Using a rubber spatula, gently press down the batter and dough to fill the pan.
• Using an offset spatula or butter knife, carefully drag the tip through the batter and dough to marble the two together. Sprinkle a bit of salt over the top.
• Bake the bars for 30-35 minutes. The bars are ready when a toothpick inserted into the center of the bars comes out with just a few moist crumbs.
• Let the bars cool in the pan on a wire rack. Once cool, pull up on the parchment paper to remove the bars from the pan. Slice into 16 squares and serve.