Whisk erythritol and cocoa powder until fine powder (removing clumps)
In a bowl, beat egg with beater or whisk to get fluffy
Add egg to erythritol and cocoa powder. Then mix in vanilla extract, salt and baking powder.
Use a mug, ramekin, or small pie dish to pour cake mixture in - spray with oil first.
Bake at 350 degrees for 10-12 minutes (or microwave for about 1 minute (check at 10 second intervals).