Preheat oven to 350 degrees.
Melt butter in a large cast-iron skillet or ovenproof baking dish, in the oven, 5 to 7 minutes
Meanwhile, place blackberries in a large bowl. Using a potato masher, mash blackberries to release their juices.
In a seperate bowl, whisk together flour, baking powder, and salt. Add 1 cup sugar, milk, and vanilla extract; mix until well combined. Remove skillet from oven and add melted butter to flour mixture; stir to combine. Pour mixture into skillet and pour blackberries and their juices into the center
Place the skillet into the oven and bake until golden brown and a cake tester inserted into the batter comes out clean, about 1 hour. Serve hot, and dont forget the ice cream - oh my!