Sift the powdered sugar into a large bowl to remove any lumps.
Melt the butter in the microwave in 30-second bursts, then pour it into the bowl with the powdered sugar, along with the milk and vanilla.
Stir everything together with a spoon until smooth and combined. Add more liquid if you would like a thinner consistency.
Notes
This glaze recipe uses melted butter for richer flavor, smoother mouthfeel, and to help it set up a little bit firmer. This is totally optional, though, for a more traditional powdered sugar glaze, leave the butter out and add enough milk to reach the desired consistency. The glaze will thicken slightly as it cools down. If it becomes too firm while you are using it, you can put it in the microwave for a few seconds to loosen it up again.