1. Melt the butter in a mug in the microwave. Try microwaving for 10/15 seconds, and if still solid, do 5 seconds at a time.
2. Add the cocoa powder / hot chocolate powder. I used caramel hot chocolate powder, which gave it a little more sweetness, but any is good.
3. Add the milk and whisk all with a fork in circular motions.
4. Add the brown sugar and whisk after each table spoon, until the mixture is smooth (try scraping your fork along the bottom and outside of the mug to see if there are any granules left; if not, you’re good). This shouldn’t matter too much however, as the sugar will melt in the microwave, so don’t worry if you can’t get every last granule.
5. Add the flour and again whisk with a fork until you get a smooth liquid (don’t worry if it looks a bit runny; it’s meant to be).
6. If you chose to use chocolate chips, add half of them to the mixture and mix them in. Just before you put the cake in the microwave, scatter the remainder of the chocolate chips on top.
7. Microwave the mug for 1 min 45 secs (600W), 1 min 30 secs (800W) or 1 min 15 secs (1000W).
If you want to flavour your mug cake, a good idea is to add extracts when you add the milk.