Brownie Cookies

Name: Brownie Cookies
Prep time: 15 min
Cook time: 10 min
Total time: 25 min
Cuisine: American
Ingredients: 1 cup unsalted butter (Cut into about 16 pieces (226g)); 2 oz unsweetened chocolate (chopped into small pieces (57g)); ½ cup natural cocoa powder ((50g)); ¾ teaspoon instant coffee grounds (optional); 1 ¼ cup light brown sugar (tightly packed (250g)); ½ cup sugar ((100g)); 1 large egg + 1 egg yolk (room temperature); 2 teaspoons vanilla extract; 2 ¼ cups all-purpose flour ((263g)); 1 Tablespoon cornstarch (cornflour in UK); 1 teaspoon baking powder; ½ teaspoon baking soda; ¾ teaspoon salt; ½ cup semisweet or dark chocolate chips ((255g) plus additional for topping, if desired)