Preheat the oven: Preheat your oven to 350°F (175°C).
Prepare the cake batter: In a large bowl, combine the all-purpose flour, sugar, cocoa powder, baking powder, and salt. Mix well until combined.
Add wet ingredients: Pour in the milk, melted butter, and vanilla extract. Stir until the mixture forms a smooth, thick batter.
Incorporate nuts (optional): If you’re using chopped walnuts, fold them into the batter gently.
Transfer the batter: Pour the cake batter into a greased 9x9-inch baking dish, smoothing it into an even layer.
Prepare the caramel topping: In a small bowl, mix together the brown sugar and an additional 1/4 cup of cocoa powder. Sprinkle this mixture over the top of the cake batter evenly.
Add hot water: Carefully pour the hot water over the cake batter (do not stir!). The water will settle at the bottom, creating a delicious sauce while the cake bakes.
Bake the cake: Place the baking dish in the preheated oven and bake for about 35-40 minutes, or until the top looks set and the edges are firm.
Cool and serve: Let the cake cool slightly before serving. Enjoy it warm, topped with a scoop of vanilla ice cream or whipped cream for that extra touch of heaven.