Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish.
In a mixing bowl, combine the flour, sugar, baking powder, and salt. Whisk until well blended.
Add the milk, melted butter, vanilla extract, lemon juice, and lemon zest to the dry ingredients. Stir until just combined; the batter will be lumpy.
Pour the batter into the prepared baking dish. Do not stir.
In a separate bowl, mix the water and remaining sugar until dissolved. Slowly pour this mixture over the batter in the baking dish. Again, do not stir.
Bake for 40-45 minutes, or until the top is golden brown and the edges are bubbling.
Let it cool for about 10 minutes before serving.