Preheat the oven to 400°F. Line a baking sheet with parchment paper and set it aside.
In the bowl of a food processor fitted with a metal blade, combine the flour, Dutch process cocoa powder, sugar, baking powder, baking soda, and salt.
Process the dry ingredients with six 1-second pulses, or until evenly combined.
Remove the cover from the food processor. Sprinkle the chilled diced butter evenly over the dry ingredients. Add the grated lemon peel.
Cover and process with twelve 1-second pulses, or until the butter is cut into small pieces throughout the flour mixture.
In a small bowl, whisk together the chilled buttermilk, egg yolk, and vanilla extract until fully blended.
Transfer the flour and butter mixture to a large mixing bowl.
Add the miniature semisweet chocolate chips and stir until evenly distributed.
Pour the buttermilk mixture into the bowl. Stir gently just until the dough begins to form. Do not overmix.
Transfer the dough to a clean countertop and gently shape it into an 8-inch circle.
Using a sharp knife or pizza wheel, cut the dough into 8 wedges.
Place the wedges on the prepared baking sheet.
Brush the tops lightly with milk and sprinkle evenly with turbinado sugar.
Bake for 18 to 20 minutes, or until the tops of the scones are crusty and the scones are baked through.
Transfer the scones to a wire rack and allow them to cool for at least 5 minutes before serving.