Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large mixing bowl, combine the flour, granulated sugar, cocoa powder, baking powder, and salt. Mix well.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients. Stir until just combined; do not overmix.
Pour the batter into the prepared baking dish, spreading it evenly.
In a separate bowl, mix the brown sugar and remaining cocoa powder. Sprinkle this mixture evenly over the batter.
Carefully pour the hot water over the top of the cake batter. Do not stir; the water will create a fudge layer as it bakes.
Bake in the preheated oven for 40-45 minutes, or until the top is set and a toothpick inserted in the center comes out mostly clean.
Let the cake cool for about 10 minutes before serving. Serve warm, ideally with a scoop of vanilla ice cream on top.