If the discard pile is frozen and you have two natural cards in your hand matching the top card of the frozen pile, can you take just the top card or must you take the entire pile?
You always take the entire discard pile, whether it is frozen or not does not affect this. There is no way in the game to only take the top card without taking the rest.
From the rules:
Under certain conditions, instead of drawing from the stock, you are permitted to take the whole of the discard pile. In order to do this, you must be able to meld the top discard, without needing any of the other cards in the discard pile to make your meld valid.
Also a point on strategy, the phrase "you must take the entire pile" implies that you wouldn't want to. While it's certainly possible to think of a situation in which you wish you could take just the top card; in practice you will almost always be happy to take the entire discard pile; as it will generally contain lots of cards that you can use to meld and score points.