Instructions

 Adjust oven rack to middle position and heat oven to 425 degrees. Line baking sheet with parchment paper. Grate 8 tablespoons butter on large holes of box grater into bowl; transfer to freezer until needed. Whisk milk and sour cream together in second bowl and refrigerate until needed.
 Whisk flour, brown sugar, baking powder, baking soda, and salt together in third bowl. Add frozen grated butter and toss with your fingers until thoroughly coated. Using rubber spatula, gently fold in chilled milk mixture until just combined.
 Transfer dough and any floury bits to well-floured counter. Using your lightly floured hands, knead dough gently 6 to 8 times until it just holds together in ragged ball, adding flour as needed to prevent sticking.
 Roll dough into 12-inch square. Gently fold top, bottom, and then sides of dough over center to form 4-inch square, loosening dough from counter with bench scraper if necessary. Transfer dough to lightly floured plate and freeze for 5 minutes; do not overchill.
 Transfer dough to floured counter and roll again into 12-inch square. Sprinkle chocolate chips evenly over dough and press lightly to adhere. Loosen dough from counter with bench scraper, roll into tight log, and pinch seam closed. Turn dough seam side down and press flat into 12 by 4-inch rectangle. Using floured bench scraper, cut dough crosswise into 4 equal rectangles, then cut each rectangle on diagonal into 2 triangles. (Triangles can be refrigerated for up to 24 hours; bake as directed.)
 Place scones on prepared sheet, brush tops with melted butter, and sprinkle with Demerara sugar. Bake until tops and bottoms are golden brown, 18 to 25 minutes, rotating sheet halfway through baking. Transfer scones to wire rack and let cool for at least 10 minutes. Serve warm or at room temperature.