The Butter Foundation: Preheat your oven to 350°F. Pour the ½ cup of melted butter directly into the bottom of a 9x13-inch baking dish, spreading it evenly.
Mix the Batter: In a medium bowl, whisk together the self-rising flour, 1 cup of sugar, the milk, and the vanilla extract until smooth.
The First Pour: Pour the batter directly over the melted butter. Do not stir. Let the batter sit naturally on top of the fat.
Prep the Berries: In a separate bowl, gently toss the 3 cups of fresh or frozen blackberries with the remaining ½ cup of sugar until the berries are lightly coated.
The Second Layer: Spoon the sugared blackberries evenly over the top of the batter. Once again, do not stir.
Bake: Place the dish in the preheated oven and bake for 45 to 55 minutes. You are looking for the cake crust to turn deep golden brown and the edges to bubble with fruit juices.
Serve: Let it cool slightly so the syrup thickens, then scoop it warm into bowls.