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Add the butter and peanut butter to a microwave-safe bowl and cook on High for 30 secs to melt. Remove from the microwave and stir thoroughly. Allow to cool slightly then add the egg and stir to combine. Grease with butter and line a second microwaveable round dish (about 15cm wide) with cling film. Tip the peanut butter mix into the lined dish then sieve in the cocoa powder and flour and tip in the sugar. Mix gently to form a thick, sticky paste. Smooth the top of the mixture with the back of a large spoon.
Using a teaspoon, make crater-holes in the top of the brownie mixture and fill each generously with jam as it will melt and bubble down when cooked. Pop in the microwave for 3-4 mins on High. Remove and leave to cool for 15 mins (it will carry on cooking as it cools) then enjoy!