Place the butter in a jug and melt in the microwave* (takes about 30-60 seconds). Stir in the peanut butter until combined.
Add the flour, baking powder, sugar, milk, vanilla extract, salt, eggs and 4 ½ tbsp of the jam to the jug. Whisk to combine.
Pour the mixture into 3 microwave-proof mason jars or mugs. Add ½ tbsp of jam to the top of each jar.
Microwave for 90-110 seconds per jar– until the cake rises over the rim of the mug (keep a close eye on it). Then let it sink back down for a few seconds, before giving a quick (10-20 second) blast or two again. This should help to ensure the cake is cooked in the middle
Remove from the microwave. Test if it’s cooked all the way through (microwave for a little longer if not).
Leave to cool for a few minutes (be careful, the microwaved jam will be very hot, so this needs to cool). Sprinkle with sifted icing sugar and chopped peanuts before serving.
* My microwave is 900w - so your mug cake may take a little more or less time depending on the wattage..Nutritional Information is per mug.