There are loads mug cake recipes all over the internet, but personally i think egg whites cooked in the microwave taste weird and many of my attempts either overcooked or undercooked or simply exploded out of the mug!
After a bit of trial and error, here's a good basic recipe to try:
2 tbsp marg
2 tbsp sugar
1 egg yolk
3 tbsp plain gf flour
1 tsp choc chips
Beat together in mug with a fork.
Cook for 1 min in microwave on high.