In a 12-ounce microwave-safe mug, using a small hand whisk or a fork, mix the almond butter and egg until smooth. This will take some time - about 2 minutes until completely smooth.
Add the sweetener, 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 on a microwave-safe plate (to catch any spills) and place it 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, 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. The mug will be hot, so use oven mitts and place the mug on a heatproof trivet.
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Notes
How Long to Microwave
As with all microwave cakes, you will need to experiment to find exactly how long you should microwave the cake in your microwave and using your mug. My microwave is 1200 watts, and I typically use a 12-ounce mug or an 11-ounce bowl. Try to stick with a similar-sized mug. As for how long to microwave, 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 better to err on the side of microwaving too little than too much. You can always add more 10-second sessions, but you can't fix a dry cake.
Whisk the flour, cocoa powder, and baking powder in a 12-ounce mug until smooth and free of lumps. It's helpful to use a mini hand whisk. Whisk in the egg whites and sweetener. If using coconut flour, let the mixture sit for two minutes (the coconut flour will absorb the liquids), then remix. Use a small rubber spatula to ensure there's no dry residue at the bottom of the mug, especially if using coconut flour. Scatter the chocolate chips on top. Microwave for 40-50 seconds (in a 1200-watt microwave) or until done to your liking. Approximate nutrition for the almond flour version: 170 calories, 10 grams carbs, 4 grams fiber, 11 grams fat, 10 grams protein.Approximate nutrition for the coconut flour version: 120 calories, 11 grams carbs, 5 grams fiber, 5 grams fat, 9 grams protein.
Oven Instructions
Bake the cake in a preheated 350°F oven for 12 minutes, then check to see if it needs more time in the oven. Make sure to use a greased oven-safe ramekin and place it on a square of foil (or on a rimmed baking sheet) to catch any spills.
A Few More Notes
If using a single-serve almond butter packet, after you squeeze out as much of the almond butter as you can, cut the edges open with scissors and use a small rubber spatula to scrape out the remaining butter. Otherwise, you’ll get around 20 grams of almond butter instead of 32 grams.
The FDA recommends cooking eggs thoroughly. This is an advantage of using liquid egg whites - they are pasteurized.
The nutrition info assumes that a sugar-free sweetener was used.