Preheat the oven to 350° F (176° C). In a 9” square glass baking dish, add the flour, salt, baking powder, granulated sugar, and cocoa powder. Fluff to aerate and stir to combine fully.
Heat the water in a two cup glass measuring cup in the microwave and set aside.
In a small bowl, add the brown sugar and cocoa powder and mash with a fork to combine well, breaking up larger chunks of brown sugar. Set aside.
Pour the milk and melted butter into the dish, stirring with a fork to create the cake batter. Spread the batter evenly in the dish then sprinkle the nuts atop if using.
Pour the brown sugar/cocoa powder on top of the cake batter and spread evenly. Set the baking dish close to the oven.
Carefully pour the hot water into the dish on top of the brown sugar/cocoa mixture. Open the oven and slowly place the dish in, careful not to splash the water around.
Bake for 45 minutes until the pudding is bubbling up around and through the cake. The cake will look dark and possibly cracked.
Serve warm with whipped cream or ice cream. Reheat portions or the whole dish in the microwave.