Put the butter in a MWable recipient.. (NO METALS!!)
..so we can melt it down in the MW. You'll get about 1/4 cup, 2 oz or 50mL, which ever serves you best.
Get the egg..
..and add everything up (ideally you'd want to add and mix each ingredient separately in this order: egg, butter and sugar, salt and flour, cocoa, vanilla)
Mix everything with a spatula. Do it thoroughly so you get no sugar nor flour crums
Get the fudge-ish dough into your cup or preferred MWable small recipient (no metals!!)..
..and turn it on for 1-minute (if it doesn't get well cooked, try 2 or 3 minutes, 'cause it depends on your MW oven)
You can decorate with sugar powder, sprinkles, nutella, etc. I put some peruvian manjarblanco on it (like fudge with no chocolate because it tastes like caramel). ENJOY!