1- First start by putting the sugar in a mixing bowl.
2- Mix in the cocoa, flour, baking powder and vanilla.
3- Now just add the milk, oil, nutella and eggs and mix really well with a wooden spoon.
4- Put the mixture in 3 medium size mugs or 2 large mugs.
5- The original recipe called for 1 and a half minutes in the microwave. When I tried the recipe, the cake needed 3 minutes to be ready. I think it depends from one microwave to another. My suggestion is to start with 1 and a half minute and check the cake. If it doesn't seem ready, add 30 seconds, and so on until you are satisfied with how it looks.