Put the butter into a large, microwave-safe mug. Microwave for 10–20 seconds, or until the butter has melted.
Once the butter is melted, use a fork to stir in the brown sugar, caster sugar, self-raising flour and cocoa powder. Mix until well combined without any lumps.
Add the egg yolk and the vanilla extract and stir again. Microwave for 30 seconds.
After 30 seconds, remove from the microwave, taking care to protect your hands as it could be hot. Put the chocolate chips on top of the cake mixture and use a teaspoon handle to push them down slightly into the mixture. Microwave for a further 30–40 seconds.
Carefully remove from the microwave and leave the cake to cool for 15 minutes.
Eat on its own or add a topping such as a scoop of ice cream, sprinkles, a chocolate flake or strawberries. Enjoy!
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