Preheat your oven to 350°F (175°C). Pour the melted butter into a deep 9x9 inch baking dish or a deep-dish pie plate.
In a medium bowl, whisk together the flour, 1 cup sugar, baking powder, and salt. Stir in the milk until just combined, a few lumps are okay.
Pour the batter directly over the melted butter. Do not stir.
In a small bowl, mix the brown sugar, 1/4 cup white sugar, and cinnamon. Sprinkle this mixture evenly over the top of the batter. Again, do not stir.
Carefully pour the 1 cup of boiling water over the entire dish.
Slide it into the oven for 40–45 minutes. The top should be golden brown and crackly, while the center stays slightly wobbly.