Preheat oven to 350°F. Grease an 8×8-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.