Take a microwave-safe mug and add almond flour, cocoa powder, erythritol, baking powder, and a pinch of salt (optional). Stir to combine.
Crack in the egg, then add melted butter and vanilla extract. Mix well with a fork or small whisk until a smooth batter forms.
Optional: Stir in a few sugar-free chocolate chips for extra richness.
Microwave on high for 60–90 seconds. Check at the 60-second mark; if the center is still wet, microwave it for an additional 15–30 seconds.
Let the cake rest for 2 minutes before eating.
Enjoy or top with sugar-free whipped cream, nut butter, or berries!