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

In a large bowl, whisk together the flour, sugar, baking powder, and salt. Add the cold, cubed butter. Using a pastry blender or your fingertips, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with a few pea-sized pieces remaining.

In a separate small bowl, whisk together the 3/4 cup of heavy cream and the egg yolk. Pour this mixture into the dry ingredients and gently fold until just combined. Do not overmix. Gently fold in the finely diced peaches.

Turn the dough out onto a lightly floured surface. Gently pat or roll the dough into a 3/4-inch thick circle (about 8 inches in diameter). Cut the circle into 8 equal wedges, like a pizza.

Place the wedges on the prepared baking sheet. Brush the tops lightly with the remaining 1 tablespoon of heavy cream and sprinkle lightly with extra sugar, if desired.

Bake for 15-18 minutes, or until the tops are golden brown and a toothpick inserted into the center comes out clean.

While the scones cool slightly, prepare the glaze: Whisk together the powdered sugar, milk, and vanilla extract until smooth. Drizzle generously over the warm scones.