Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish with butter or nonstick spray so the cake releases easily.
In a large bowl, combine the flour, sugar, 1/4 cup cocoa powder, baking powder, and salt. Stir thoroughly so the leavening and cocoa are evenly distributed.
Add the milk, melted butter, and vanilla extract to the dry mix. Stir until everything comes together into a smooth batter. Scrape the bowl to incorporate any streaks of flour.
If you’re adding chopped nuts, gently fold them into the batter now for added crunch.
Pour the batter into the greased 9x9-inch baking dish, spreading it evenly with a spatula to the corners.
In a separate bowl, mix the brown sugar and the additional 1/4 cup cocoa powder, and sprinkle this mixture evenly over the batter in the dish.
Carefully pour the hot water over the top of the cake batter without stirring.
Bake in the preheated oven for 35-40 minutes, or until the cake has risen and the top looks set.
Let the cake cool slightly for 10 minutes before digging in. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream.
Notes
Serve with vanilla ice cream, whipped cream, or toasted nuts for crunch. A dusting of powdered sugar or a drizzle of caramel elevates it into a celebratory treat.