For a quick meal + drink at The Cosmopolitan, Holsteins does the trick. Big caveat up front is that we dined at 11 pm and I didn't have a burger per se but still, it was a decent meal.
Because it was that late, service was speedy and attentive. The in-house DJ was a little loud though which made conversations a little tough. I really only needed a snack so I ordered a crispy pork belly slider (http://www.yelp.com/biz_photos/P7pxQFqr7yBKMMI2J51udw?select=iFmsSniZf-3AgCH-ZYZakA), or tiny buns as they call them here. The fatty pork belly had a nice char on it and the pillowy steamed buns (http://www.yelp.com/biz_photos/P7pxQFqr7yBKMMI2J51udw?select=MCPdx_nHJ4JSD0a_3LLQ3w) served as a great receptacle to handle said meat. I thought there was a heavy hand on the hoisin sauce but all in all it was a pretty good combination. My friend had the carbonara chicken burger (http://www.yelp.com/biz_photos/P7pxQFqr7yBKMMI2J51udw?select=VQxHmfVC8yd988No1srqUw) and he thought it was alright.
They do have a good selection of beers here though. I started with the Tripel Karmeliet from Brouwerij Bosteels on tap and it was a crisp triple with hints of citrus, florals, and ummm banana (to me at least). Next I had a Leffe, also on tap. I thought the Leffe tasted a little off... as though it was slightly diluted but it might have been a result of the Karmeliet before.
All in all, if you want an elevated meat between two buns plus good booze, Holsteins is a good choice if you're at the Cosmopolitan.