Preheat your oven to 375 F / 190 C.
In a small bowl mix the cocoa powder, powdered sweetener, baking powder, and salt.
In a small bowl, beat your egg until a little fluffy. The air in the egg is needed as there is no flour.
Combine the two mixtures together along with the heavy cream and vanilla extract.
Wet a large mug with a little oil, and fill with cake batter, place on baking tray and bake for 10 – 15 minutes, the top of the cake should be firm but jiggly.
Another option is to microwave your mug cake for around 60 seconds. Remember, each microwave is different, so after 60 seconds, check on the mug cake and if it isn't solid, put it back in for 10-second intervals until done.