Beat the butter and Marmite together until smooth, then form it into a log on a piece of baking parchment, roll up and twist the ends like a Christmas cracker. Chill in the fridge until you need it. Will keep for up to a week in the fridge or two months in the freezer.
Boil the sprouts for 3-4 mins, then drain and leave to steam-dry. Heat a non-stick frying pan, then add the sprouts and dry-fry them for 4-5 mins or until they start to blacken on the cut sides and at the edges. Take the pan off the heat, then add a chunk of the Marmite butter (keep the rest for another day). Sizzle until the butter has melted and shake the pan gently to coat the sprouts. Season with pepper, but taste before adding salt.