Mix all the wet ingredients in a mixing bowl.
Add the sugar, followed by the flour and cocoa powder. Mix well.
Lastly, add the chocolate.
Pour into oversized mugs, filling them to about three-quarters full.
Microwave each on high for 3 minutes, letting them rest in the microwave for at least 5 minutes before removing them.
TIP: Serve this as a warm pudding: remove from the mug and drench in cream or custard.
*Buttermilk can be replaced with full-cream milk, melted salted butter or sunflower oil