Melt the butter and chocolate in a microwave-safe container at 600 watts in the microwave. Then add the remaining ingredients and stir well. Now cook in the microwave for 10 to 15 minutes (stick test) at 600 watts. Then let it rest for another 5 minutes.
When choosing the container, please make sure that it is high enough, as the cake will rise a lot. Grease the pan in which you bake the cake beforehand.