Mix the dry ingredients into a microwave-safe mug/bowl. I used a mug with a diameter of 4 inches/10 cm (the volume is about 1 cup/0.25l).
Mix the wet ingredients in a separate bowl before pouring into the mug or pour directly into the mug to mix. Mix well with a mini whisk or spoon/fork, to thoroughly combine. You want a smooth, lump-free batter.
Add part of the chocolate chips and stir them into the mix. Then sprinkle the rest on the top of the cup.
Microwave on high for 1 minute (my microwave's high power is at 900W). If you prefer a slightly 'underdone' lava middle then you could try to microwave it for 40-45 seconds initially. If you find that your cake needs a little longer even after 60 seconds then place back into the microwave for a further 10-second burst.Best served immediately.
To Make It In The Oven
Simply make sure that it is in an oven-safe dish, like a ramekin. Bake in the oven at 180ºC/350ºF for between 10-15 minutes. You can check to make sure that it is cooked through with a toothpick or skewer.I'd suggest watching it quite carefully after the 10-minute mark though as over-baked mug cake isn't great.
Make sure to stir/whisk the chocolate mug cake batter thoroughly. Otherwise, you may end up with little pockets of flour or flour at the bottom of the microwave mug cake- which no-one wants!
If you have issues with the mug cake spilling over at all, then simply use a larger mug/dish.
You can (optionally) grease your mug, to avoid any sticking. Though I tend to skip this step.
Microwave cook times may vary depending on the make and model of your microwave. For my own microwave, I've found that 45-50 seconds yields a slightly gooey center still (which some may prefer). Whereas 60 seconds seems to be the perfect amount to be cooked, without being overcooked.