Preheat the oven to 350 degrees. Coat a 9 x 9-inch square baking pan with cooking spray and set aside.
In a medium mixing bowl, whisk together the flour, sugar, baking powder, cocoa powder, and walnuts.
In a small glass measuring cup, melt the butter in a microwave oven for 10 seconds on high. Add the milk to the butter and heat in the microwave for 45 seconds.
Pour butter/milk mixture over the flour mixture, add vanilla, and stir well to mix. Spoon cake batter into the prepared baking pan.
In a small mixing bowl, crush together the brown sugar and cocoa powder. Sprinkle the mixture over the top of the cake batter.
Bring 1 cup of water to a boil (using the microwave or stove top or electric kettle). Pour the water over the cake batter.
Bake for 30-35 minutes and serve warm. A little vanilla ice cream on the side couldnt hurt.