Need help to recreate

I went to a brew pub that had an amazing burger that I would like to try to recreate it for friends.  It started with three different cuts of beef ground up to male the patty, white cheddar on the bottom of the patty and this custom sweet butter they made.  The pairing of the cheese and this sweet butter concoction (no, it wasn't just normal sweet butter) was amazing.  I asked what it was and they said it was made with their stout beer.  I'm looking for an idea of what I can do to try and replicate this - doesn't necessarily need to be made with the stout nbeer,but something to make this a really sweet butter to go with the cheddar.