In a medium mixing bowl, whisk together the dry ingredients: flour, sugar or xylitol, baking powder, and salt.
Add the water, lemon juice, your chosen fat (butter, oil, or mashed banana), lemon zest, and vanilla extract. Stir well until the batter is smooth and all ingredients are combined.
Lightly grease a microwave-safe mug or a large ramekin. Pour the prepared batter into it.
Microwave on high for about 1-2 minutes, or until a toothpick inserted into the center comes out mostly clean. (Alternatively, you can bake it in a preheated oven at 350°F (175°C) for about 10 minutes.)
Let the mug cake cool for a minute, then enjoy!