Preheat your oven to 350°F (175°C).

In a large mixing bowl, combine the flour, granulated sugar, baking powder, and salt. Mix well.

Stir in the milk, melted butter, and vanilla extract until fully combined.

Pour the batter into a greased 9x13 inch baking dish, spreading it evenly.

In a separate bowl, mix the cocoa powder and brown sugar together.

Sprinkle the cocoa mixture evenly over the batter in the baking dish.

Carefully pour the hot water over the top of the cocoa and batter mixture. Do not stir.

Bake in the preheated oven for 30 to 35 minutes, or until the top is set and a toothpick inserted comes out mostly clean.

Let it cool for a few minutes before serving.