In a small saucepan set over low heat, melt together 3 oz of the bittersweet chocolate and the butter. Once melted, transfer into a large bowl and allow to cool for ten minutes.

In a separate medium or large bowl, whisk together the flour, cocoa powder, and baking powder and set aside. Finely chop the reserved (1 oz) of bittersweet chocolate and set aside.

Gradually whisk the eggs into the cooled melted chocolate mixture. Add the sugar and mix until well incorporated. Slowly add the flour mixture, and stir with a wooden spoon or spatula until well combined - the batter will be thick. Gently fold in the chopped chocolate, cacao nibs and fleur de sel (or regular salt if you are using that instead).

Preheat your oven to 350° F and line a cookie sheet with parchment paper or a non-stick baking mat.

Using spoons or cookie scoop, portion out rounded teaspoons of batter. Using your hands, shape them into balls and place them on the prepared cookie sheet. Make sure you leave at least a one-inch space between them.

Place the cookie sheet in the refrigerator for ten minutes. Bake in the oven for 8-10 minutes or until the tops feel just dry to the touch. Do not overbake.