In a medium pot, combine blackberries, 1 cup sugar, and water. Bring to a boil, reduce heat to a simmer, and cook for 5 to 10 minutes until reduced and thickened, stirring occasionally. Remove from heat.
While blackberries are cooking, whisk together flour, remaining 1/2 cup sugar, baking powder, and salt in a large bowl. Slowly blend in milk and vanilla and mix until batter is smooth.
Put butter (sliced into 6 equal pieces) in a 9 by 13-inch baking dish and place in preheated oven for a couple of minutes to melt. Remove dish from oven and tilt so that butter evenly coats the bottom. Slowly pour batter over melted butter. Gently spoon berries and syrup evenly over batter without stirring.
Bake for 30 to 35 minutes or until cobbler is light golden brown and tests done with a toothpick inserted in the center. Serve warm with vanilla ice cream or fresh whipped cream.
A 16-ounce bag of frozen blackberries (thawed) may be substituted for fresh.
The blackberries will start to break down when they are simmered. If you prefer some whole blackberries in your cobbler, you may simmer only 3 cups of berries with the sugar and water and then stir in the remaining 1 cup of berries after removing from the heat.
Blackberries are typically pretty tart, but if you use particularly sweet blackberries, you may reduce the sugar used in the simple syrup.
The blackberries and syrup are to be spooned on top of the batter. However, the berries will sink through the batter and then partially rise as the cobbler bakes. This is why you see the blackberry layer at the bottom of the cobbler in the photos.