Preheat your oven to 415 degrees Fahrenheit and line a baking sheet with parchment paper.

In a medium mixing bowl, sift together the flour, cocoa powder, baking powder, sugar, cream of tartar, and sea salt.

Cut the chilled butter into the dry ingredients using a pastry blender, your hands, or a knife and fork until the mixture resembles coarse crumbs.

Add the milk and stir with a fork until the mixture comes together; then, turn the dough out onto a lightly floured surface and knead until smooth.

Dust your countertop with cocoa powder, roll out the dough to 1/2 inch thick, and cut out biscuits using a 3-inch cutter or into squares; re-roll any scraps to make more biscuits.

Arrange the cut biscuit dough on the prepared baking sheet and bake for 10-12 minutes, or until they are risen.

Serve the chocolate biscuits immediately while warm, or set them aside until ready to serve.