Preheat oven to 350 degrees.
Melt butter in a 10-inch enameled cast-iron skillet over medium heat.
Meanwhile, place blackberries in a large bowl. Using a potato masher, mash blackberries to release their juices. If blackberries are tart, sprinkle with sugar.
In another large bowl, whisk together flour, baking powder, and salt. Add sugar, milk, and vanilla extract; mix until well combined. Add melted butter to flour mixture; stir to combine. Pour mixture into skillet and add blackberries and their juices into the center. Sprinkle with sanding sugar.
Transfer skillet to oven and bake until top is golden brown and a cake tester inserted into the batter comes out clean, about 1 hour. Serve warm or at room temperature with creme fraiche, whipped cream, or ice cream.