If your mug is big enough, you can mix this right in the mug, but I mixed it in a small mixing bowl instead. You want a large, microwaveable mug, one that will hold around 1 cup. This will rise and then go down as it cools, so you don't want a small mug where it will overflow!
Place the flour, sugar, cocoa powder, salt, bicarbonate of soda and chocolate chips into your bowl and whisk briefly until combined and the chocolate chips are coated in flour.
Add in the oil, milk and vanilla and whisk until smooth and there are no lumps of flour.
Spoon the mixture into your mug and place in the microwave for 60-90 seconds. My microwave is 850W and it took 75-80 seconds. I placed the brownie in the microwave for 60 seconds, then sprinkled a few more chocolate chips on top for the final 15-20 seconds.
The brownie in a mug is done when the top is just set and springs back slightly when you touch it. Also, an inserted skewer/knife comes should come out clean. You still want the brownie to look soft and gooey. Don't keep microwaving until it looks dry and solid.
You can dig straight in, but it will be VERY hot. I like to let mine stand for 2 minutes, then add some non dairy ice cream. Brownie in a Mug is best eaten as soon as it's made, but will keep covered in the fridge overnight. Just reheat for 30-40 seconds when you want to serve it.