In a large bowl, whisk together the gluten free flour, granulated sugar, 2 tablespoon of Dutch cocoa powder, 1 tablespoon of dark cocoa powder, baking powder and the salt.
Very slowly whisk in the milk and vanilla into the melted butter. Do this very slowly otherwise the mixture will get chunky and the milk with curdle from the hot butter.
In a medium bowl, combine the dark brown sugar, the remaining 2 tablespoon Dutch cocoa powder and the remaining 1 tablespoon dark cocoa powder. Sprinkle evenly all over the top of the batter.
Very carefully put the pan in the center rack of the oven. Bake for 60 minutes. When you take it out of the oven you might notices some bubbling; that's normal.