In a large bowl add the blackberries, brown sugar, salt, cornstarch, vanilla extract, and lemon juice. Tip: if using frozen blackberries, be sure to thaw and drain beforehand.
Use a large spoon to mix everything together thoroughly. Set aside.
Begin making the crust. In a large bowl, add the flour, baking powder, salt, and sugar. Use a whisk to mix the dry ingredients
Then add in the cold cubed butter and mix it in with a pastry blender. Cut into the butter until it comes down to the size of a pea. Tip: to make this step faster you can do this in a food processor.
Pour in the cold buttermilk and mix it in with a spoon until no more patches of dry flour are visible.
Add the sugary blackberries to a 9x13-inch baking dish. Then preheat the oven to 350 degrees F.
Use a medium-sized cookie scoop to add dollops of the biscuit topping right over the blackberries. Now brush the melted butter on top of the biscuit dough and sprinkle some coarse sugar on top (optional).
Bake for about 1 hour or until it's bubbling and the biscuit topping is golden brown.