The single biggest food rule for exam week is: do not try anything new. Your favorite spicy hotpot on the night before the exam is a perfectly reasonable ordinary decision that could ruin your morning.
Eat the meals you always eat, at the times you always eat them. Familiar bowls of rice, familiar noodles, familiar vegetables. This is not the week to experiment.
Breakfast on exam day should include some protein (egg, milk, soy milk) and some slow carbohydrate (steamed bun, congee). Avoid heavy oil — it slows you down.
Do not drink so much liquid at breakfast that you need the bathroom mid-exam. Sip through the morning instead.
Chocolate is not a magic exam food. Bananas are not either. Eat what you have always eaten, in slightly smaller portions than usual.