Can an energy drink or chocolate before an exam be useful ? What sort of other edible goods be helpful?
I think doing such things may be risky. Instead, do something natural. The most important is to get plenty of sleep the night before the exam and don't try to cram up to the last minute. 

Between the two exams parts, get some light exercise and, again, don't cram. Let your mind relax and let it integrate what you know rather than trying to force it to absorb things that may mask other, more important things. 

As for food, eat as you normally do but nothing heavy that will make you groggy and wanting an afternoon nap. If you eat differently, your gut may rebel against you. 

If you have been taking good notes throughout the learning period, then summarizing them on note cards (big ideas only) in the week previous can be a good plan. You can then do a light review at the morning and mid-day meal. A light review. Big ideas. 

But make your body and mind as comfortable as possible. Breathe deeply. Relax your face. Smile. (These are martial arts advices, actually.)