In a large mixing bowl combine flour, baking powder, salt, sugar and lemon zest. Whisk well. Add 12 tablespoons of well chilled butter in ½” cubes. Cut butter into dry ingredients until mixture resembles a rocky road. In a separate bowl combine milk, sour cream, lemon juice eggs and vanilla. Whisk well and add to dry ingredients. Fold mixture together gently, taking care not to over mix.