Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper or lightly grease it.

Prepare the fruit: In a medium bowl, gently toss the diced peaches and nectarines with 1/4 cup sugar, cornstarch, and vanilla extract. Set aside while preparing the batter.

Combine dry ingredients: In a large bowl, whisk together the flour, 1/3 cup sugar, baking powder, and salt. Add the cold, cubed butter. Using a pastry blender or your fingertips, cut the butter into the flour mixture until it resembles coarse crumbs the size of small peas.

Mix wet ingredients and combine: In a separate small bowl, whisk together the buttermilk, beaten egg, and lemon zest. Pour the wet mixture into the dry ingredients. Gently fold until just combined, being careful not to overmix. The dough will be shaggy.

Fold in fruit: Gently fold the prepared peach and nectarine mixture into the batter until evenly distributed. The fruit juices will slightly color the dough.

Drop and Bake: Using a large ice cream scoop or two spoons, drop mounds (about 1/2 cup each) of dough onto the prepared baking sheet, leaving about 2 inches between them. Sprinkle the tops generously with coarse turbinado sugar, if using.

Bake for 22-25 minutes, or until the tops are golden brown and a toothpick inserted near the center comes out clean (or with bubbling fruit juices clinging to it). Let cool slightly on the baking sheet before transferring to a wire rack.