Preheat the oven to 350 degrees F and butter a 10-inch cast-iron skillet.
Whisk together the flour, baking powder, salt, 1 cup sugar, the melted butter and milk in a large bowl. Beat well, then pour the batter into the buttered cast-iron skillet. Top with the fresh blackberries. Sprinkle with the remaining cup sugar, then pour the boiling water over the top.
Bake until the filling is bubbling and the cobbler topping is golden, 35 to 40 minutes. Pour sassy sauce over the top before serving.
Yield:enough for 1 cobbler
Whisk together the sweetened condensed milk and the heavy cream in a saucepan over low heat until combined and warm. Spike with rum. Pour over the cobbler.