Line baking sheets with parchment paper and set them aside. Make sure the butter is softened but not melted.

Beat the softened butter and confectioners’ sugar until smooth and creamy, taking care not to whip the mixture excessively. Mix in the vanilla extract.

Whisk the all-purpose flour, cocoa powder, and fine salt together in a separate bowl. Add the dry mixture to the butter mixture in batches, mixing only until a cohesive dough forms.

Fold in the dark chocolate chips or chunks until they are evenly distributed. Shape the dough into a log approximately 2 inches (5 cm) in diameter.

Wrap the dough log tightly and refrigerate it for 30–60 minutes, or until firm enough to slice cleanly. Meanwhile, heat the oven to 350°F (175°C).

Slice the chilled dough into rounds about 1/4 inch (6 mm) thick. Arrange the rounds on the prepared baking sheets, leaving about 1 inch between cookies.

Bake each tray for 10–12 minutes, until the cookies look set but are not noticeably browned around the edges. Let them cool on the baking sheet for 5 minutes.

Transfer the cookies to a wire rack and cool completely. They will firm as they cool, giving you a tender center and a delicate shortbread snap.