Whisk together the almond flour, erythritol, baking powder, and salt in a small bowl.
Add the lemon juice, melted butter, egg, and lemon zest until well combined.
Lightly grease a mug then pour the batter in.
Microwave on high heat for 1 minute and 30 seconds until cooked through.
Serve the mug cake hot with whipped cream, if desired.