The bar expansion you can buy in the cafe is?

In the cafe, you can incrementally add 1 energy at a time to your drink by tapping on the shopping bag icon bottom left of the cafe. It is the second offer from the left. There is no bar involved.

Outside of the lounge cafe, there is an extended energy bar that you can purchase for 799 diamonds, which allows your energy to refill to 150 instead of 110, thereby allowing you to be away from the game for 5 hrs (instead of 3hr20m) and still accrue energy. That offer is located by tapping on the "+" sign next to your energy count.