7.
Add the vanilla and confectioners’ sugar and whisk to combine. Whisk in the milk, 1 tablespoon at a time, until the mixture is creamy and smooth. If the frosting seems too thick, add a little more milk. (You can keep the frosting in a covered container in the fridge if you make it ahead of time or aren’t quite ready to use it. When you’re ready to fill the cookies, give the frosting time to soften a bit, or stick in the microwave for 10 seconds or a bit more.)