Butter and flour a baking sheet.
Sift 2 cups of flour, 1/3 cup sugar, baking powder, baking soda, and salt into a large bowl.
Add butter, rub in with fingertips until it is reduced to the size of rice grains. Mix in chocolate chips.
Whisk buttermilk, egg yolk, and vanilla in a small bowl to blend. Add mixture to dry ingredients; mix until dough comes together in moist clumps, and gather wet dough into a ball.
Press dough out on lightly floured surface to 8-inch round. Cut round into 6 wedges.
Transfer wedges to prepared baking sheet, spaced 1 inch apart.
Preheat oven to 400°.
Brush scones lightly with milk, sprinkle with remaining 2 tbsp sugar.
Bake until scones are crusty on top and tester inserted into center comes out clean, about 15-20 minutes. These are a soft scone. If you like a firmer scone, cover with foil at 10-15 minutes to keep from getting too brown and cook an additonal 5-10 minutes.
Serve warm with whipped or clotted cream.