Preheat the oven to 190C/375F. In a medium bowl combine the dry ingredients; flour, cocoa powder, baking soda and salt.
In the bowl of a stand mixer, fitted with a paddle attachment, cream the butter with the brown and white sugar until light and fluffy, then beat in the egg and vanilla.
Add the mixed dry ingredients to the butter and sugar mixture and start combining them on low speed. Stop once in a while, scrape the sides with a flexible spatula and continue beating until the dough forms. Mix in the chopped mint flavoured chocolate and give everything a brief mix until the chocolate is well incorporated into the cookie dough but do not overmix. We don't want to break the chocolate.
Scoop the dough with a small cookie scoop and place the dough balls on parchment or silicone lined cookie sheet 1 inch apart as they will spread in the oven. Bake in the preheated oven for 8-10 minutes or until hard around the edges. Take out of the oven and sprinkle with chopped chocolate or press a few chocolate chips into warm and soft cookies, then top with the flaked sea salt.
Let the cookies cool for 5 minutes, then transfer on wire rack to cool completely.
Notes
If you are not able to find mint flavoured chocolate, use regular chocolate +1 tsp peppermint extract flavouring.