For the filling: Preheat the oven to 375 degrees F. Lightly grease an 8-inch square baking dish or 9-inch cast-iron skillet with butter.
Put the berries in a large bowl, then add the granulated sugar, flour, cinnamon, vanilla, salt and butter and toss together until well coated. Pour into the baking dish. Set aside.
For the batter: Whisk together the flour, granulated sugar, baking powder, salt, butter and egg in a bowl to form a stiff batter. Drop the batter by the spoonful over the prepared berries, then bake until golden and bubbling at the edges, about 35 minutes. Let cool slightly before serving. Serve with whipped cream.