Preheat oven to 400°. Line a baking sheet with parchment paper; set aside.
Combine 1 3/4 cups plus 3 tablespoons flour, cocoa powder, 1/3 cup sugar, baking powder, baking soda and salt in the bowl of a food processor fitted with a metal blade. Process with six 1-second pulses.
Remove cover of food processor and sprinkle the butter evenly over the dry ingredients. Add the grated lemon zest. Cover and process with 12 1-second pulses.
In a small bowl, whisk buttermilk, egg yolk and vanilla to blend.
Transfer dough to a large bowl. Mix in chocolate chips. Stir in buttermilk mixture until dough begins to form.
Transfer the dough to a countertop and shape into an 8-inch circle.
With a sharp knife or pizza wheel, cut the dough into 8 wedges.
Place wedges on an ungreased baking sheet and brush scones lightly with milk; sprinkle with turbinado sugar.
Bake in preheated oven 18 to 20 minutes or until tops of scones are crusty. Transfer to a wire rack for at least 5 minutes before serving.