Preheat the Oven: Preheat the oven to 375°F.
Prepare the Nectarines: Peel the nectarines and slice them until you have approximately 6–8 cups of fruit.
Cook the Fruit Filling: Place a large skillet over medium heat and melt 1 tablespoon of butter, if using. Add the sliced nectarines, ⅓ cup sugar, and cornstarch. Stir and cook for 7–8 minutes, or until the fruit releases its juices and the mixture begins to thicken. Add the Vanilla: Stir the vanilla extract into the cooked nectarine mixture until evenly incorporated.
Transfer the Filling: Pour the warm nectarine filling into a 2- to 2½-quart baking dish, spreading it evenly across the bottom.
Combine the Dry Ingredients: In a large mixing bowl, combine the flour, 1 cup sugar, baking powder, and salt. Stir until the ingredients are evenly distributed.
Cut in the Cold Butter: Add the cubed cold butter to the flour mixture. Use a pastry blender, fork, or your fingertips to work the butter into the dry ingredients until the mixture resembles coarse crumbs.
Make the Topping Dough: Pour in the heavy cream or buttermilk. Mix until the ingredients come together to form a cohesive dough.
Add the Topping: Drop tablespoon-sized portions of the topping dough over the nectarine filling. Leave some spaces between the portions, as the topping will puff up during baking.
Sprinkle with Sugar: Generously sprinkle sparkling sugar or granulated sugar over the topping for a sweet finish.
Bake the Cobbler: Place the baking dish in the preheated 375°F oven and bake for approximately 38–45 minutes, until the topping is light golden brown and the fruit filling is bubbling. If using a smaller baking dish, additional baking time may be necessary.
Serve Warm: Allow the cobbler to cool slightly, then serve warm with vanilla ice cream or homemade whipped cream.
