New York Black and White Cookies

Name: New York Black and White Cookies
Prep time: 45 min
Total time: 45 min
Cuisine: American
Ingredients: 128 grams all purpose flour (1 cup); 1 teaspoon cornflour/cornstarch; 1/4 teaspoon baking powder; 1/4 teaspoon kosher salt; 1 large egg, at room temperature; 1 teaspoon vanilla extract; 60 grams unsalted butter (4 tablespoons), softened; 70 grams granulated sugar (1/3 cup); 60 milliliters buttermilk (1/4 cup); 270 grams confectioner’s sugar (2 cups); 30 milliliters water (2 tablespoons), plus more to adjust consistency; 20 grams light corn syrup (1 tablespoon); 1 teaspoon lemon juice; 20 grams unsweetened cocoa powder (3 tablespoons)