Preheat oven to 350°F (175°C) and lightly grease a 9×13-inch baking dish.
In a bowl whisk together flour, granulated sugar, baking powder, salt, milk, melted butter, and vanilla until smooth.
Pour batter evenly into the prepared baking dish.
In a separate bowl mix brown sugar and cinnamon together.
Sprinkle the cinnamon-sugar mixture evenly over the batter.
Carefully pour hot water over the entire mixture without stirring.
Bake for 40–45 minutes until the top is golden brown and mostly set.
Let cool slightly before serving to allow the sauce to thicken.