Butter and flour baking sheet. Sift 2 cups flour, 1/3 cup sugar, baking powder, baking soda and salt into large bowl. Add butter and lemon peel; rub in with fingertips until butter is reduced to size of rice grains. Mix in chocolate chips. Whisk buttermilk, egg yolk and vanilla in small bowl to blend. Add buttermilk mixture to dry ingredients; mix until dough comes together in moist clumps. Gather dough into ball. Press dough out on lightly floured surface to 8-inch round; cut round into 6 wedges. Transfer wedges to prepared baking sheet, spacing 1 inch apart. (Can be prepared 1 day ahead. Cover and refrigerate.)

Preheat oven to 400°. Brush scones lightly with milk; sprinkle with remaining 2 tablespoons sugar. Bake until scones are crusty on top and tester inserted into center comes out clean, about 20 minutes. Serve warm.