Melt butter in a medium (microwave safe!) bowl. Whisk in cocoa and sweetener to taste, and mix until well combined. If using erythritol, your batter will likely be very thick.
Whisk in the egg and mix until smooth (if using xylitol and allulose) or until thoroughly combined (with granular erythritol, which doesn't dissolve as readily). Mix in the rest of the ingredients and spoon the (very thick!) batter into a mug.
Place a paper towel into the microwave and set the mug on top (to catch any overflowing batter). Cook on high for 70-90 seconds (time can vary a lot in microwaves, but to give you an idea I do 80 on 1000 watts!). Allow to cool for a couple minutes and enjoy!
*Please see section on sweeteners for deets and possible substitutions. If using allulose (which is 70% less sweet) stick to the upper suggested range. And keep in mind that keto sweet buds vary drastically, so feel free to play around with the sweetness that suits you... but 2 TBSP works great for me! Please note that a batch yields a (very large!) mug cake (think the equivalent of two muffins!). So feel free to portion out (think half post dinner and the rest for lunch!).