In a free standing mixer, cream the butter, sugar, vanilla and salt until pale and fluffy.

Add the egg to the mix, and continue mixing.

Add the Cocoa powder, flour and chocolate. Mix on a low speed just until it has combined, make sure not to over mix otherwise the cookies will become tough.

Roll into 16 balls. Place on a tray lined with baking paper, then chill for a couple hours or overnight.

Preheat the oven to 170 degrees. Flatten the balls slightly and place on a tray, making sure they are spaced out enough as they do spread a lot!

Bake for about 8-10 mins. The key is to not over bake them, it is better to be slightly under as you want them to be soft and chewy. They should have hardened on the outside, but still lovely and soft on the inside.

Once they are out of the oven, tap the tray on a hard surface to make them lovely and flat.

When they are cool, drizzle dark chocolate over the top and sprinkle some sea salt flakes over the cookie.