Preheat oven to 200C/180C Fan/Gas 6. Line 2/3 large baking sheets.
In a large bowl, or the bowl of a stand mixer, combine the butter and sugar until fully creamed (use an electric mixer or the paddle attachment on medium 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 choc 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 - they spread A LOT! Don't worry if your cookie dough looks a little runnier than standard cookie dough - it's what makes these so decadent! Scatter the reserved chocolate over the top.
Bake for 15 minutes. Remove trays and leave to cool. Repeat for remaining batter. They will continue to cook a little once out of the oven, so don't wait until they get too dark!