In a large bowl, whisk together flour, baking powder, baking soda, salt, and sugar. Toss in cubed butter. Cover with plastic wrap and let chill in the freezer for 15-20 minutes.
Spread chopped peaches in a single layer over a parchment lined baking sheet. Place in the freezer for 15-20 minutes, until semi-frozen.
In a small sauce pot, melt ¼ cup butter over medium heat. Once melted, swirl pot occassionaly to ensure even heating. Mixture will bubble, crackle, and foam as it cooks. The bubbling will be vigorous at first, but it will subside. Continue to cook until melted butter smells nutty. By this point, the milk solids in the butter will have turned golden brown in color. Remove from heat and pour into a heat-safe container or bowl. Be sure to scrape out as much of the browned milk solid bits. Set aside and allow to cool for 15 minutes.
Preheat oven to 375°F. Line baking sheet with parchment paper or silicone mat. Set aside.
Remove dry ingredients and peaches from the freezer. Using a pastry blender (pastry cutter) or two forks, cut butter into flour until mixture resembles coarse sand.
In a small bowl, whisk together milk, sour cream, and egg until smooth. Add wet ingredients to flour mixture. Add 1 Tablespoon of liquid brown butter. Fold to combine until mixture is a "shaggy mess." Gently fold in cold peaches.
Dump coarse dough onto a floured work surface. Gently knead dough together using a bench scraper (or your hands). Pat dough into a circle with a diameter of 8-inches and thickness of about 1/2-inch. Cut dough into 8 equal pieces.
Transfer scones to prepared baking sheet, allowing at least 2-inches of space between scones. Brush scone tops with remaining liquid brown butter. Sprinkle turbinado sugar on top.
Bake for 20-22 minutes until scones have puffed up and the tops are lightly golden brown in color. Rotate pan midway through baking to ensure even cooking. Let scones cool slightly on baking sheet. Brush once more with remaining liquid brown butter. Transfer to wire rack to cool. Serve scones warm or at room temperature.