Mix the filling
Preheat the oven to 375°F(190°C). Have ready an 8-inch (20-cm) square baking dish. In a large bowl, gently stir together the nectarines, raspberries, and 3 tablespoons of the sugar. Pour into the baking dish, spreading evenly.
Mix the topping
In a small bowl, whisk together the buttermilk and vanilla. In a large bowl, mix together the flour, ¼ cup (2 oz/60 g) of the sugar, the baking powder, the baking soda, and the salt. Using a pastry blender or 2 knives, cut in the cold butter until the mixture forms coarse crumbs. Add the buttermilk mixture and stir gently until a soft dough forms. Drop the dough by heaping spoonfuls onto the fruit, spacing it evenly. Brush the dough lightly with the melted butter, then sprinkle with the remaining 1 tablespoon sugar.
Bake the cobbler
Bake until the filling is bubbling, the topping is browned, and a toothpick inserted into the center of the topping comes out clean, about 35 minutes. Let cool for 15 minutes on a wire rack. Serve warm with vanilla ice cream, if desired.