Place the butter in a 350ml mug and microwave for 10-20 seconds until melted

Add the egg, milk and vanilla to the mug and beat in with a fork until combined. Add the sugar, flour and salt and beat again until smooth.

Cook in the microwave for 1 minute 25 seconds @600W, 1 minute 15 seconds @800W or 1 minute 5 seconds @1000W. Leave to cool.

Spoon the jam over the cake. Ready to serve!