My partner opened a "strong" one NT. I am holding:
♠ void
♥ KJxx
♦ AKQxxx
♣ KJx
What should I bid?
I would bid 4C (Gerber) to ask about aces. If partner has 3, bid 7D. If partner has only 2, bid 6D.
The weak point in this hand is if partner has, say, the KJ in spades; some other responder might have a fancier convention to pick out the specific high cards.
Now that this is open again:
Opposite a strong 1 NT, I bid 3D if it is natural, or I transfer to diamonds if that is available. If partner denies support for diamonds I bid 6D. If he shows support for diamonds I investigate 7D with what-ever methods are available to me. Consider where partner's 16 points are. An "unfortunate" hand from partner would be this: AKJx AQx xx Qxxx, and 6D is looking pretty icy even with all those wasted values in spades. In a practiced partnership I investigate grand slam even if partner denies diamond support initially.
Opposite a weak NT I will probably still end up in 6D, though I might blind the auction by jumping there in an unpracticed partnership. Why help the defence by bidding precisely when it is going to be of more help to the defence?
You have to make a forcing bid, what you specifically bid will depend entirely on what conventions you and your partner are playing with.
Assuming you're playing with a strong 1NT opener (15-17 HCP), that means you and your partner have 32-34 HCP. Since it's 33 points for slam and you've got a void, you know you've got that. That means you have to force the bidding to continue until you get to the 6 level. Whether or not you make it go to 7, and in which specific contract you want to end up, those are things you can use all that bidding room to decide. A typical tool for this purpose is Blackwood/Gerber. With a bid suit preference, I like the additional information gathered with Roman Key Card.
Here's a nice wiki reference to help you pick a Blackwood convention to use.
Just keep in mind that unless you've got that 33 points (pretty rare) there's a good chance you're just bidding yourself into a hole. I've definitely seen players who are way too anxious to try out their Blackwood bidding!