BEFORE STARTING: I think it is best to chill your mixing bowl in the freezer, as well as your whisk attachment. The colder everything is to start out, the better. I usually leave mine in for a few hours or overnight.
Pour the heavy cream into the mixing bowl, and begin whisking on high. Add the vanilla (you can add a little more if you'd like, to ensure you really taste vanilla), then gradually add the powdered sugar. Stop your mixer every once in a while to see if it's sweet enough. Start with small amounts, you can always add more, but you can't take it away if there's too much.
After you have figured out how sweet you want it, whisk on high until your cream starts to thicken. I don't know for sure how long I usually whisk it, but I know it's at least a couple minutes. Keep an eye on it, because you can overmix. Overmixing will cause a strange, almost sticky consistency.
After whipping it up nice and fluffy, store it in the fridge. It will keep for a few days, and it makes a good amount, so you have some time to finish it or bake more things to put it on. :)