I was going through some stuff when I found an almost-untouched can of Tudor Gold Cocoa Powder (hence the 5 Minute Chocolate Mug Cake).
This is not the cheap stuff that come in sachets, this is pure soluble cocoa, hence it needs to be (and should be) treated differently when you want to make hot chocolate using this.
Get a mug, put one tablespoon of sugar, one rounded teaspoon of cocoa powder (you may want to use more but for the first time, start with one) and two teaspoons of water. Stir the sugar, water and cocoa until the cocoa is completely wetted out.
Add about one inch of milk and stir to completely mix your cocoa paste and milk. Fill the rest of the mug with milk, up to about 1/2 an inch off the top. The mix will expand about 5% on heating, so don't over-fill.
Put the mug in the microwave, heat for about 2 minutes on High.