Preheat oven to 450°F.
In a medium bowl, whisk together the flour, baking powder, granulated sugar, cinnamon, and salt.
Pour in the buttermilk. Stir until mostly combined, being careful not to overwork the dough.
Gently fold in the drained chopped peaches. (Make sure the peaches are well drained; the extra liquid can affect cooking time.)
In a small bowl, combine the melted butter, brown sugar, and cinnamon.
Pour the melted butter mixture into an 8×8-inch baking dish. Place the dough on top of the butter. Using a spatula (or your hands), evenly spread the dough out to the edges of the pan.
In a small bowl, combine sugar and cinnamon. Sprinkle 1 tablespoon of the mixture over the biscuits.
Score the unbaked dough into 9 equal squares (3×3 pattern). Bake for 30-35 minutes, or until the the biscuits are golden brown.
After removing the biscuits from the oven, sprinkle the remaining cinnamon sugar mixture on top (optional).
Serve.