1. Preheat oven to 350°F (176°C). Line a baking sheet with a silicone baking mat or parchment paper.
2. Cream butter and sugars together until light and fluffy, about 3-4 minutes.
3. Mix in egg and vanilla extract.
4. Add flour, cocoa, baking soda and salt and mix until well incorporated and thick.
5. Stir in the chocolate chunks.
6. Roll into balls of cookie dough about 2 tablespoons in size.
7. Place the cookie dough balls on the lined cookie sheet. Bake for 8-10 minutes. The center might look a little undercooked, but the edges should be done. The centers will cook a little more and firm up as the cookies cool.
8. Remove cookies from oven and allow to cool for 2-3 minutes, the remove to cooling rack to cool completely.