Preheat oven to 350 degrees F. Lightly grease an 8-inch or a 9-inch square pan or a small casserole dish.
In a large bowl, thoroughly mix together the flour, sugar, salt, baking powder and 2 tablespoons of the cocoa powder.
Combine the milk, butter, and vanilla in a large glass measuring cup. Pour over flour mixture and stir just until combined (don't beat). Scrape batter into the pan.
Mix the remaining sugar, brown sugar and cocoa powder together and sprinkle over the top. Pour the boiling water over all. Set on center rack and bake 35-45 minutes. A wider pan such as a 9-inch square should take around 35 while the 8-inch pan takes between 40 and 45.