Heat oil in a large saucepan. Tip in the corn, cover and shake the pan to coat the kernels. Cook over a medium heat until the corn stops popping, about 5 mins, shaking the pan every so often. Take off heat and sprinkle with a little salt.
Put cheese, mustard and pepper in a small pan and heat gently until melted and bubbling. Drizzle over the popcorn and mix well until completely coated.
Rub hands with a little oil and quickly grab handfuls of popcorn and squeeze into brain shapes. Place on a tray lined with parchment, then leave to cool. Cover with cling film until ready to eat or store in a jar – you can make a few hours before serving.