Add all ingredients into a microwave-safe mug (I recommend using a mug that can hold at least 10 oz of liquid as the cake will initially rise quite high before sinking back down). Vigorously mix with a mini whisk until batter is completely smooth. I highly recommend using a mini whisk rather than a fork, as you really need to thoroughly mix the batter just the same as any cake batter. You do not want any egg streaks to go unmixed.
Cook in the microwave at full power for about 1 minute 15 seconds. Top of cake should look mostly done but may still be slightly gooey around the edges. Let cake cool a few minutes before eating. If desired, you can top with frosting, syrup or more spread.
Notes
If you follow a gluten-free diet, double check to make sure the cashew butter brand you are using is gluten-free. I used Jif brand, but can't say for sure that it is gluten-free.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.