When she sobers up, she can swear you raped her.
Do you really want that drama?
You could get it in writing but people have gotten out of contracts because they were intoxicated when they signed.
"He got me drunk and forced me to sign"
In other words, you are always taking a chance.
Your best shot is to have her sign a witnessed agreement when she is sober, and then get drunk.
Have her sign a document stating:
Being of sober mind, I am completely okay with (your name) getting me drunk, leading to sex. Signed: (her signature and dated)
If she's smart she'll add to the agreement: Offer expires in 24 hours from date signed.
Otherwise you could use it any time you want. She might be okay with that, some girls are, especially if you look like one of the Hemsworth brothers,