Soften the butter until it is room temperature and workable.
In a large mixing bowl, add the butter and milk. Mix them lightly to combine, and start adding the sugar a tablespoon at a time.
Use electric beaters to make the job easier. Continue adding sugar as it dissolves into the butter.
If after 1 cup you haven't reached the desired consistency, continue adding more sugar a tablespoon at a time. It should get fluffy and strong enough to stand up to being piped into rosettes.
Once fully whipped up, add the vanilla extract and blend in thoroughly. You could also add some food colouring at this stage, if you wanted.