Grease a large, microwave-safe mug with cooking oil spray, and set it aside. Instead of in a separate bowl, the batter can be mixed right in the mug. That just makes it more difficult to whisk properly.
In a small bowl, place the nut butter, applesauce, egg, honey, cocoa powder, salt and baking powder, and whisk to combine well. The mixture should be thick and may be a bit lumpy, which is fine.
Transfer the batter to the prepared mug. You can, of course, whisk the batter right in the mug you plan to cook it in. It may be a bit harder to mix well, though.
Place the mug in your microwave oven and cook at about 850 watts for 30 seconds. (See Recipe Notes)
Remove the mug from the microwave, and stir the batter.
Return it to the microwave, and cook again at about 850 watts for about 1 minute more, or until the cake is mostly set. It’s fine if the cake seems a bit wet in the center, which is preferable to overcooking it.
Allow the cake to cool briefly before serving with whipped cream and berries, or plain.
Video
Notes
About microwave wattage.
It is important to be able to approximate 850 or 900 watts of power in your microwave oven. My microwave is 1200 watts, so I set it to 70% power for this recipe.