Mash the butter and marmite together until a smooth paste is formed.
Coat both sides of the bread in the marmite butter (you may have some leftover, so it depends on your taste how much you use), heap the cheese onto one face of the bread, with a hefty grind of black pepper and chives (if using). Pop the other slice of bread on top and transfer to a cold frying pan.
Turn the heat on low, and press down on the top of the sandwich with something heavy or a spatula for the first few minutes of cooking. Cook for 3-4 minutes on each side, keeping the flame low, until you have a gorgeous golden crust on each side and the cheese looks fully melted.