Preheat oven to 375℉/180℃. Line 2 large baking sheets.
In a large bowl, or the bowl of a stand mixer, combine the butter and sugar until fully creamed (about 3 minutes on high speed). Then add the egg and vanilla and mix again.
Add the flour, cocoa powder, baking soda and the pinch of salt! Stir together until just combined. Add in the chocolate chunks (or use chocolate chips, either are fine!). Reserve some chocolate to sprinkle on top prior to baking
Use a tablespoon or small ice cream scoop to scoop the batter onto the trays - approximately 3/4 cookies per tray, at least 3 inches apart they spread a good amount! Don't worry if it looks a little runnier than standard cookie dough. Scatter any extra chocolate over the top.
Bake for 15 minutes, when the edges look crisp (they will continue to bake a little once removed from the oven). Let cool. Repeat for all remaining batter.