Preheat oven to 180°C (350°F). Line a baking sheet with parchment paper and set aside.
Melt butter in the microwave in 15-30 second intervals until melted. Allow to cool slightly.
Into a bowl, add flour, cocoa powder, salt, and baking soda. Mix until well incorporated and set aside.
In the bowl of an electric mixer fitted with the paddle attachment, add brown sugar, white sugar, and melted butter.
Beat on medium speed for 2-3 minutes until well incorporated and most of the sugar has dissolved.
Add the egg and vanilla. Beat for another 30 seconds.
Add the flour mixture in two batches, mixing on the lowest setting just until incorporated.
Fold in the chocolate chips.
Using two spoons, spoon the batter onto the lined baking tray to make 10 cookies.
Sprinkle with sea salt flakes.
Bake for 9 minutes. The center of the cookie will still be soft but will harden slightly when cooled.
Allow the cookies to cool for 12-15 minutes on the baking tray, then transfer to a cooling rack.