Preheat oven to 350°F. Grease an 8x8-inch square baking dish.
In a large bowl, whisk together the flour, baking powder, salt, granulated sugar, and 2 tablespoons cocoa powder.
Stir in the milk and melted butter until smooth. Gently fold in the nuts.
Pour batter into the greased baking dish.
In a medium bowl, mix together the brown sugar and remaining 4 tablespoons cocoa powder.
Sprinkle the brown sugar mixture over the cake batter. Pour the hot water over the top.
Bake for 40–45 minutes. Cool for 10 minutes.
Cut into squares. Invert squares onto plates and spoon sauce from the pan over each one.
Top with whipped cream or vanilla ice cream.
Notes
Hot water is true to the original recipe. Using coffee instead of the water will add even more flavor to the finished cake. You can also use hot water + a teaspoon of espresso powder. Store the cake for 4–5 days at room temperature or in the refrigerator, tightly covered.