How do you cook lamb butterball?

"Lamb butterball" is not a standard culinary term. It's possible that you're referring to one of the following:

1. A whole lamb shoulder, rolled and tied: This is sometimes called a "butterball" because the shoulder is shaped like a ball after it's rolled and tied.

2. A lamb leg that's been deboned and rolled: This is another possibility, though less common than a whole shoulder.

To clarify, could you please describe the cut of lamb you're working with? For example, tell me:

* What kind of cut of lamb is it? (shoulder, leg, etc.)

* How is it shaped? (is it rolled, tied, or just a single piece?)

Once I understand what you're working with, I can give you more specific instructions on how to cook it.