FOR THE COOKIES: – 2 & 1/4 cups (270g) all-purpose flour – 1 & 1/2 teaspoons cornstarch – 1 teaspoon baking soda – 1/2 teaspoon salt – 3/4 cup (170g) unsalted butter, softened – 3/4 cup (150g) firmly packed light brown sugar – 1/2 cup (100g) granulated sugar – 2 large eggs, room temperature – 2 teaspoons pure vanilla extract – 1 & 1/4 cups mini semi-sweet chocolate chip FOR THE FILLING: – 1 cup (226g) unsalted butter, softened – 1 cup (190g) shortening – 2 pounds (907g) confectioners’ sugar – 2 teaspoons vanilla extract – 1 & 1/2 tablespoons hot water – sprinkles, optional