1. Preheat the oven to 350 degrees F. Melt the stick of butter in a large cast-iron skillet or ovenproof baking dish in the oven (takes about 5 to 7 minutes).
2. Place half of the blackberries in a large bowl and mash them with a potato masher to release some of the juices. Add the rest of blackberries and toss. If the berries are tart, sprinkle them with additional sugar (up to approximately 1/4 cup).
3. To make the batter, in another bowl, whisk together the flour, baking powder, and salt. Add the 1 cup sugar, milk, and vanilla extract, and stir until evenly blended. Remove the skillet from the oven and add the melted butter to the batter and stir to combine. Pour the batter into the skillet, then add the blackberries and juices to the center of the batter.
4. Bake until the top is golden brown and a toothpick inserted into the batter comes out clean, approximately 1 hour. Serve warm with whipped cream or ice cream.
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