Cut cold butter into 1 Tbsp-sized pieces and then cut in half again. Transfer to a plate and set in the freezer to chill until ready to use.
Preheat oven to 375F and line a cookie sheet with parchment paper. Set aside.
In large bowl or in food processor, whisk or pulse together flour, cocoa powder, sugar, baking powder, and salt.
Remove butter from freezer and cut in to dry mixture* until dough becomes like coarse crumbs.
In a small-medium bowl, whisk together heavy cream, egg and vanilla until combined.
Gradually stir into scone mixture until dough is moist and beginning to cling together**, transfer to a lightly floured surface and gently knead, kneading in chocolate chips by hand, just until cohesive ball begins to form. Break into equal halves.
Mold each half of the dough into about an 8-inch wide circle and cut each into 8 wedges.
Transfer wedges to parchment paper-lined baking sheet and bake for 18-20 minutes. Allow scones to cool 10 minutes before glazing.
Whisk together your sifted powdered sugar and cocoa powder.
Whisk in milk and vanilla extract until completely combined and smooth.
Immediately drizzle over scones.
If you want to eat immediately the glaze will be a bit messy, allow to sit for 10-20 minutes to allow glaze to set.
*If using a pastry cutter and not a food processor, take care to press straight down as you cut the butter into the flour mix. **If using food processor, take care that you do not overwork and break your machine (this is a lot of dough). Once the dough begins to clump, transfer to your lightly floured surface and knead it into disks there.