In a large mixing bowl, beat together the softened butter and sugar until the mixture is creamy and just beginning to look fluffy.
Add the egg and vanilla extract to the bowl, and whisk until well incorporated, with a slight sheen on the surface.
In a separate small bowl, whisk together the flour, cocoa powder, baking soda, and salt to distribute the leavening and flavor evenly.
Gradually add the dry mixture to the wet ingredients, stirring gently until the dough is just combined and slightly sticky.
Fold in the chopped dark chocolate chunks, ensuring they are evenly distributed throughout the dough.
Scoop generous spoonfuls of dough onto a lined or greased baking sheet, spacing them about 2 inches apart for spreading.
Bake the cookies in a preheated oven at 350°F (175°C) for 10-12 minutes, until the edges are crisp and set, with a slightly soft center.
Remove the cookies from the oven and allow them to cool on the baking sheet for 5 minutes, letting their edges harden while the centers stay fudgy.
Transfer the cookies to a wire rack to cool completely, which helps them develop a crunchy exterior with a tender inside.
Once cooled, enjoy these dense, crackly-edged cookies with a glass of milk or your favorite beverage.
