Place the sweetener, almond flour, coconut flour, cocoa powder, and baking powder in a 4-cup glass mixing bowl. Stir to combine.
Add the mayonnaise, egg, water, and vanilla extract. Stir until the batter is smooth.
Divide the batter between two 4 ounce ramekins or 1-16 ounce microwave mug cake cup.
Place the cake batter in the microwave and cook on high power for 2 minutes. If cake is still wet on top, microwave in 10 second intervals until no longer wet.