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!
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