Cookies: 2 and 1/4 cups (280g) all-purpose flour (spooned & leveled) 1 teaspoon baking soda 1 and 1/2 teaspoons cornstarch* 1/2 teaspoon salt 3/4 cup (170g / 12 Tbsp) unsalted butter, melted & cooled 5 minutes 3/4 cup (150g) packed light or dark brown sugar 1/2 cup (100g) granulated sugar 1 large egg + 1 egg yolk, at room temperature 2 teaspoons pure vanilla extract 1 and 1/4 cups (225g) semi-sweet chocolate chips or chocolate chunks Double Chocolate chip muffins 1 3/4 cups (219 g) all-purpose flour, spooned and leveled 1/2 cup (40 g) Dutch process cocoa powder 2 1/2 tsp baking powder 1/4 tsp baking soda 1/2 tsp salt 1/2 cup (112 g) unsalted butter, melted 1 cup (200 g) granulated white sugar 2 eggs, at room temperature 1/2 cup (120 ml) whole milk, at room temperature 1/2 cup (122 g) sour cream, at room temperature 2 tsp vanilla 1 cup (200 g) mini semi-sweet chocolate chips 1/2 cup (100 g) mini semi-sweet chocolate chips for sprinkling on top of the muffins