Preheat oven to 350 degrees. Spray a 8 x 8 square or round pan with non-stick cooking spray. Melt the stick of butter in the pan, in the oven as it preheats. In a medium bowl, mix milk, flour, sugar, baking power and salt with a wire whisk. Pour over the melted butter and drop the blackberries evenly into the batter.
Bake 1 hour. During the last 10 minutes of baking, sprinkle a tablespoon of sugar on top of the cobbler.