2Place butter into the bottom of a 9x13-inch baking dish and place in the oven until butter is melted, about 5 minutes. Whisk flour, sugar, milk, and baking powder into a smooth batter in a bowl. Pour batter over melted butter without stirring.
3Mix blackberries with blackberry jam in a separate bowl and spoon blackberries and any accumulated juice evenly over the batter.
4Bake in the preheated oven until blackberries are bubbling and cobbler is set, about 1 hour.