FOR THE COOKIES: – 2 & 1/4 cups (270g) all-purpose flour – 1 tablespoon cornstarch – 1/2 teaspoon baking powder – 1/4 teaspoon baking soda – 1/2 teaspoon salt – 1 cup (110g) confectioners’ sugar – 1/2 cup (113g) unsalted butter, softened – 1 large egg, room temperature – 1/4 cup full fat sour cream, room temperature – 1 & 1/2 teaspoons vanilla extract FOR THE FROSTING: – 1 cup (226g) unsalted butter, softened – 1 tablespoon vanilla extract – 1/4 teaspoon salt – 3 tablespoons heavy cream, warm but not hot – 4 cups (440g) confectioners’ sugar, sifted – gel food coloring (optional) – sprinkles