Preheat the oven to 350 degrees F. Butter a 11x13 baking pan or 12 inch Dutch oven.
In a large bowl combine the sugar, flour, baking powder, salt and cinnamon.
Whisk in the milk and butter until smooth. Pour the batter into the baking dish or Dutch oven.
Evenly top the batter with the blackberries.
Bake for about 50 minutes, then remove from the oven and sprinkle on the brown sugar. Continue to bake for about 10 minutes or until golden brown.
Let cool slightly before serving to allow the cobbler set up a bit.