Very close to Potsdamer Platz, on Stresemannstrasse, you can find Solar. It’s located on the 16th floor of a building, is itself on 2 floors. “Downstair” is a restaurant I never tried, looks quite pricey, and on the 2nd level is the cocktail lounge. Most of the tables are turned towards the big windows, offering you a very nice view on Berlin’s skyline.
On week-ends, it can get pretty packed and it’s hard to get a table, unfortunately, it’s not possible to book one. It’s recommended to dress up a little, especially for guys on friday and saturday evenings. There’s a garderobe at the entrance of the building (if I recall, last time I went it was mandatory, but can’t remember whether you have to pay for it or not) before getting in a lift situated on the side of the building to take you directly to the restaurant, to get to the bar you have to cross the dining room and take the stairs.
The place can be hard to find, you have to get into the courtyard between the Hotel and Pit-stop, the building is in the back. I’d recommend you get to the S Bahn station Anhalter Bahnhoff and follow the exit to Stresemannstr, it’s almost directly ahead of you then.