Line a 22cm/8inch square cake tin with parchment/greaseproof paper

Put the peanut butter and cubed butter into a large microwave-proof bowl and cook on high for 30 seconds

Stir the mixture and microwave again for another 30 seconds

Stir in the vanilla extract and chocolate before adding the icing sugar. The consistency should resemble cookie dough

Using a spatula, put the mix into the lined cake tin pressing it down to level it out. Use the back of a spoon if needed

Cover the tin with cling film or foil and refrigerate for at least 4 hours

Remove from the tin and slice into squares. Keep refrigerated in an airtight container