You have AA in the BB. The BTN opens to 3.5x. What is your line ?
The standard line would be to re-raise to what ever your defend re-raise is.
Depending on how often the BU raises you should be defending with a re-raise a decent range. If you defend with 88+ and sometimes just suited connector then AA will not stick out. Just call with some of your re-raise range sometimes but I would always re-raise AA here.
Give them a chance to re-raise you back in case they have a real hand like KK so you can get it all in. I have read never slow play the nuts as your opponent might have a hand willing to come along.
I believe my line would be Woohoo! in my best Homer Simpson voice, likely accompanied with a few fist pumps.
(Note: This likely works best in online play, where your opponent will not see/hear you.)
I would then make my standard 3-bet for the game situation.
When you have AA, your ultimate goal is to get all the stacks in the middle pre-flop, so always RAISE when you have the option pre-flop with AA.