Preheat oven to 180ºC and line 2 baking trays with parchment paper.
Cream the butter and sugars together in a stand mixer until combined.
Add the egg and vanilla extract and beat well, followed by the baking soda and cocoa powder.
Add in the choc chips and beat on low speed until just combined (don't overwork the mixture or it can become tough).
Use a cookie scoop to dollop tablespoon sized balls of cookie dough onto lined baking trays.
Sprinkle with sea salt flakes.
Bake for 10-12 minutes or until set around the edges.
Let cool for 15 minutes on the baking tray before cooling completely on a cooling rack.