Place the butter in the mug and microwave it for 10-15 seconds until it melts.
Whisk the heavy whipping cream and egg together.
Swirl the melted butter around the cup (to prevent the batter sticking later on), then pour it into the prepared egg mixture.
Mix everything together. Pour the batter into the mug and microwave for 50-60 seconds. Let it rest and serve. Enjoy the delicious carnivore chocolate mug cake!