Heat the oven to 350 and line baking sheets with parchment 15 minutes before you want to bake the cookies.
Combine the butter and the unsweetened chocolate (break into pieces) in a microwave safe bowl and heat in 20 second intervals, stirring well between each, until it is almost all the way melted. Leave sit for a few minutes and then stir until chocolate is smooth. Don't overheat it or the chocolate will clump up. You should stop heating before it is fully melted.
Whisk the sugars into the chocolate mixture and then add the eggs and vanilla and whisk well. Whisk the flour, baking soda, salt and cocoa well in another bowl. Add to the wet ingredients and mix just until combined. The dough will be thick. Fold in the chocolate chips. Cover and put into the fridge for at least 30 minutes.
Using a medium cookie scoop, scoop balls of dough and top with some sea salt if you like. Bake for 10-12 minutes until just set on the outside. You don't want to overbake these as you want a fudgy inside. If they look set on the outside before 10 minutes, take them out. Allow to cool and store in an airtight container.