While the sauce cools, make the biscuits. Sift the flour and salt together into a glass mixing bowl.
Cube the COLD butter into ½-inch cubes. Add the butter cubes to the flour and cut it in with a pastry cutter until it’s pea-sized or smaller.
Slowly add the COLD buttermilk combining it with a wooden spoon. You want it to just come together to form a dough. Don’t over-mix it!
Turn the dough out onto a floured surface. Knead the dough less than 10 times until it comes together and is somewhat smooth. Again, don’t over-knead the dough.
Roll or pat the dough into a rectangle until it is ½ inch thick.
Spread the blackberry filling over the top of the dough in a thin even layer.
Carefully roll the dough into a log so that the filling is between the layers, but not mixed in with the dough.
Cut the rolled dough into 1-inch thick biscuits using a biscuit cutter or the rim of a glass. The dough can be sticky so sprinkle some flour over it as you're rolling.
Place the biscuits into the pre-heated cast iron skillet. You want the biscuits to barely touch. This will make them rise.
Lightly brush the biscuits with a little buttermilk.
Bake the biscuits in the preheated oven for around 20 minutes unit they are golden brown.