Place the butter into your mug and pour over the boiling water. Stir until melted.
Add the cocoa powder, caster sugar and gluten free self-raising flour and mix together.
Press the dark chocolate into the centre of the mixture.
Microwave for 1 minute. The cake should be cooked on the top while staying slightly gooey in the middle.If you think it's still a little undercooked (all microwaves are very different), cook for a further 10 - 50 seconds on medium, in 15 second bursts.
Serve while warm with ice cream (optional).
Notes
RECIPE NOTES & TIPS
Gluten free self raising flour - you can use any brand of gluten free self raising flour you like - personally I find the White Wings brand is very good.
Cooking time - all microwaves vary greatly with how long they will take to cook this mug cake. I recommend cooking it for 1 minute to start, and then in 15 second extra bursts if needed.
Don't overcook the mug cake! - you want it to be slightly gooey in the middle. Stop cooking when the cake is cooked on top.
If you think it's still a little undercooked (all microwaves are very different) - cook for a further 10 - 50 seconds on medium, in 15 second bursts.
Watch the cake - watch it carefully in the microwave to make sure it doesn't overflow.
Make 2 portions - you can make this mug cake in two smaller mugs or ramekins rather than 1 large mug/ramekin.