FOR THE COOKIES: – 1/2 cup (113g) unsalted butter – 1 cup (200g) granulated sugar – 1 large egg – 2 teaspoons pure vanilla extract – 1/3 cup (75g) sour cream – 2 cups (240g) all-purpose flour – 1/2 teaspoon baking powder – 1/4 teaspoon baking soda – 1/4 teaspoon salt FOR THE ICING: – 3 cups (330g) confectioners’ sugar – 1/4 cup (59ml) whole milk – 2 tablespoons light corn syrup – 1 teaspoon pure vanilla extract – 1/2 teaspoon salt – 3 tablespoons (15g) dark cocoa powder