In a medium bowl, mix almond flour, granulated sweetener, baking powder, and salt together. Add lemon juice, lemon zest, egg, and melted butter and whisk until combined.
In a small bowl, mix powdered sweetener in water and lemon juice together to form the icing. It should be thin.
Divide cake mixture evenly between four microwave-safe mugs. Microwave on high for 90 seconds each.
Drizzle icing on top of each mug cake. Serve warm and top with fresh whipped cream and a touch of added lemon zest, if desired.