Line a baking tray with greaseproof paper and set aside.
Heat oil in a large saucepan with a pinch or two of salt.
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 minutes, shaking the pan every so often.
Once the corn has popped, remove from the heat.
Add the cheese and mustard 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 take handfuls of popcorn and squeeze into brain shapes.
Place on the baking tray and leave to cool.
Cover with cling film until ready to eat or store in a jar.
This can be made a few hours before serving.