First prepare your mug pan by greasing with butter and then dusting lightly with flour.

Next, place an iron rack (or high aluminum plate or iron gauze) into a clean empty wide pot, cover the pot really tight and preheat on high heat for 10 minutes.

Cream the butter and sugar. Using a stand mixer with a paddle attachment, beat the butter and sugar until light lemon-colored and fluffy, about 5 minutes.  

Measure the flour and crack the eggs.  While the butter and sugar are creaming, measure out the flour, baking powder, and salt into a small bowl and whisk lightly to combine. Crack the eggs into a bowl and add the vanilla.

Mix the cocoa powder with 2 tablespoons of hot tap water and powdered milk until smooth. Add the chocolate mixture to the remaining batter.  Beat to incorporate.

Then add the chocolate mixture into the batter and combine all together,

Pour into the greased mug then place carefully on the hot rack .Now cover the pot again and leave the cake to bake well on medium heat for about 5 minutes, then reduce to low heat and let the cake continue baking for 15 to 30 minutes(depending on the size of the cake you are baking).

After 15 minutes open up the pot and check for doneness by dipping the center with a skewer/toothpick.
 If the skewer comes out clean, then the cake is done baking, but if it comes out moist, let the cake bake a little longer and test with the skewer again, after 5 minutes.
 This cake took about 30 minutes. 

When the cake is done, take it out of the hot pot and leave to cool for 10 minutes before turning over