I bought this over-sized mug in the Whittard sale sale last year, especially for this "experiment". Any microwavable mug will do, although the bigger the better.
You will need:
4 Tablespoons sugar (I used golden granulated)
2 Tablespoons cocoa powder
3 Tablespoons milk
3 Tablespoons oil (I used walnut oil because I had it - bit decadent! Veg oil would also work)
Whilst in the microwave the mixture will rise way over the top of the mug - this is normal. Obviously the more narrow your mug, the more this will happen. As mine was pretty wide, when it sunk a bit upon leaving the microwave it looked like this...