Preheat oven to 400°F (200°C). Line a rimmed baking sheet with a silicone baking mat or parchment paper.
In a large mixing bowl, whisk together flour, Dutch process cocoa powder, granulated sugar, baking powder, baking soda, and salt until fully combined and no dry pockets remain.
Add cold cubed butter to dry ingredients. Using a pastry cutter or box grater, work quickly until mixture resembles coarse crumbs with pea-sized butter pieces visible.
Stir semi-sweet chocolate chips into the flour-butter mixture until evenly distributed.
In a separate small bowl or large measuring cup, whisk together heavy cream, egg, and vanilla extract until smooth.
Pour wet mixture into dry ingredients and gently fold with a rubber spatula just until a shaggy dough forms. Stop mixing as soon as no dry flour remains.
Turn dough onto a lightly floured surface. Gently knead 2-3 times to bring it together into a ball. Pat into an 8-inch circle about 1 inch thick.
Using a bench scraper or sharp knife, cut the circle into 8 even wedges. Press straight down without dragging to keep edges sharp.
Arrange wedges on prepared baking sheet 2 inches apart. Brush tops lightly with extra heavy cream. Bake for 16-18 minutes until set and a toothpick comes out clean.
Transfer scones to a wire cooling rack and cool for 10 minutes before glazing.
Whisk together powdered sugar, cocoa powder, and 2 tablespoons milk until smooth, adding more milk as needed for a pourable glaze. Drizzle or spread over warm scones and let set for a few minutes before serving.