In a large bowl, combine the flour, sugar, cocoa powder, baking powder, and salt. Mix well. 
Add the milk, melted butter, and vanilla extract. Stir until the mixture forms a smooth batter. 
If you’re using chopped nuts, fold them into the batter now. 
Transfer the batter:
Pour the cake batter into a greased 9×9-inch baking dish or a similar-sized oven-safe dish. 
Prepare the fudge topping:
In a small bowl, combine the brown sugar and additional cocoa powder. 
Sprinkle this mixture evenly over the top of the cake batter. 
Add the hot water:
Carefully pour the hot water over the top of the cake (don’t stir!). The water will sink to the bottom and create a fudge sauce while the cake bakes. 
Bake the cake:
Bake for 35-40 minutes, or until the cake has risen and the top looks set, while the fudge sauce is bubbling underneath. 
Serve:
Let it cool slightly before serving. The pudding cake is delicious served warm with a scoop of vanilla ice cream or whipped cream.
Enjoy your decadent Hot Fudge Chocolate Pudding Cake! 
