In a medium mug or a large ramekin, mix the peanut butter with the egg until completely incorporated. Check especially on the bottom to make sure there is no peanut butter residue.
If the batter seems very thick, sticky and stubborn, add a tablespoon of water and keep mixing until completely smooth.
Mix in the stevia, then the baking soda. Mix well.
Microwave the cake for 30 seconds. Check it - if it looks very wet, add 10 more seconds. I usually stop at this point - the edges are puffed and done but the center is still gooey. That's how I like it!
Enjoy the cake as is, or top it with a dollop of whipped cream for a truly rich experience.
Video
Notes
Use a freshly opened jar of natural peanut butter and stir it well. If you use a half-used jar, the peanut butter might be too dry.
Go by how the mixture looks and feels in your kitchen, with your peanut butter. I show you in the video - it should be smooth, shiny, and not too thick. If it's sticky and difficult to mix, add an extra tablespoon of water.
I don't add salt to the batter, because my peanut butter is already salted. If your peanut butter contains just peanuts and nothing else, not even salt, add a pinch of salt to the batter.
Microwaves vary. I always start with 30 seconds, check the cake, then usually add 10 more seconds. By then, the edges are puffed and the center is still gooey - and that's exactly how I like it.
You need to mix the peanut butter and egg really well until the mixture is completely smooth and uniform. If the batter is very thick and sticky, add a tablespoon of water. This makes mixing much easier.
I have made this cake in several dishes. A small and tall 10-oz mug, a wide 15-oz mug, and a 3.5-inch-wide ramekin (as shown in the video and in the photos). They all worked, and there wasn't much of a difference. Except that with the ramekin, I was worried there for a moment that the batter might overflow! But it didn't. You can always place your mug on a microwave-safe plate to catch possible spills.
Occasionally, baking soda can cause a reaction in baked goods that results in an unpleasant ammonia smell. If you're concerned about it happening, you could use 1 teaspoon of gluten-free baking powder instead of baking soda. Make sure the baking powder is fresh and not expired.