In a 12-oz microwave-safe mug, using a small hand whisk or a fork, mix the almond butter and the egg until smooth. This will take some time - about 2 minutes until completely smooth - so try to be patient.
Add the sweetener and salt (if using), whisking to incorporate.
Whisk in the cocoa powder, then add the baking powder. Mix well, making sure there's no nut butter residue in the bottom of the mug. The batter should be smooth and uniform.
At this point, the batter should fill the mug about halfway to the top. The cake will rise dramatically as it cooks, so the mug you use needs to accommodate that.
Place the mug in the microwave. Microwave it, uncovered, for 30 seconds. Check to see if the cake is still very liquid on top - if so, microwave it for 10 more seconds. It should still be very gooey and soft on top when you remove it from the microwave (see the video below), but not completely liquid. Resist the temptation to microwave more (unless the batter is still completely liquid) – a chocolate cake is always better gooey than dry.
Remove the mug from the microwave and allow the cake a few seconds to settle before digging in.
Video
Notes
As with all microwave cakes, you will need to experiment to find exactly how long to microwave the cake in your microwave and using your particular mug.I use a 12-oz mug in this recipe. Try to stick with a similar size mug. Using a much larger mug (such as a 16-oz mug) could dry out the cake. A smaller mug could result in the cake spilling out of it while in the microwave.As for how long to microwave this cake, I would start with 30 seconds, then check to see how it looks. Your goal is to keep it soft and gooey, but not liquid.It's definitely better to err on the side of microwaving too little than too much. You can always keep microwaving in 10-second increments, but you can't un-microwave a dry cake!The FDA recommends cooking eggs thoroughly.