Single Serve Chocolate Chip Cookie

Name: Single Serve Chocolate Chip Cookie
Prep time: 10 min
Cook time: 10 min
Total time: 20 min
Cuisine: American
Ingredients: 3 tablespoons unsalted butter (or salted buttered, see note 1); 3 tablespoons light brown sugar (firmly packed); 1-1/2 tablespoons granulated sugar; 1/2 teaspoon vanilla extract (optional); 1 large egg yolk (see note 2); 1/4 teaspoon baking soda; 1/2 teaspoon cornstarch (optional); 1/2 cup all-purpose flour (see note 3); 6 tablespoons chocolate chips (I love milk chocolate!); Flaky sea salt (for topping, optional)