Preheat your grill to medium heat. Lightly oil the grates or prep an area for indirect heat.
Place the peach halves cut-side down on the grill for 3–4 minutes, until grill marks appear. Remove and let cool slightly. Chop into bite-sized pieces.
In a mixing bowl, combine chopped grilled peaches, blackberries, honey (or maple syrup), lemon juice, cornstarch, and cinnamon if using. Toss gently until the fruit is well coated.
Transfer the fruit mixture into a greased 10-inch cast-iron skillet.
In a separate bowl, whisk together flour, sugar, baking powder, and salt. Cut in the cold butter using a pastry cutter or fork until the mixture resembles coarse crumbs.
Stir in milk and vanilla just until a dough forms. If too dry, add a splash more milk.
Drop spoonfuls of the biscuit dough over the fruit, leaving some space between each mound for expansion.
Place the skillet on the grill over indirect heat. Close the lid and cook for 25–30 minutes, or until the biscuit topping is golden and cooked through and the filling is bubbly.
If desired, brush the biscuit tops with melted butter for extra flavor.
Let the cobbler rest for 10 minutes before serving. Top with vanilla ice cream or whipped cream for a finishing touch.
