This is the brownie recipe I use, it is based on this recipe by Gina Homolka, except with some tweaks. I didn’t like the taste the water gave it, so I switched it out. I do think that if you replace the butter w oil, in the original recipe, you can use it for brownies in a mug. 

2-4 tbsp butter

1/3 cup sugar ( I prefer brown sugar)

1 egg

2 tbsp cocoa powder

1/2 cup flour

1/2 tsp baking powder

flavouring ( go wild. I have added a splash of coffee, cinnamon, etc.)

Pre-heat oven to 200 C

Melt the butter in a saucepan, remove from heat, stir in sugar, egg and cocoa powder. Mix until smooth, before blending in the flour, when there are no streaks, add eventual flavourings. Then, add the baking powder. Stir until the baking powder is blended in.

Grease a small pan , and pour in the batter.

Put in the hot oven for 10-12 minutes.