Preheat the oven to 350°F and butter and flour an 8X6” pan.
Melt the butter and ½ cup chocolate together in a small pan over low heat or in the microwave in 30 second increments. Stir often so the chocolate does not burn.
Whisk both sugars, the egg and yolk together in a bowl until smooth. Add in the vanilla, melted chocolate and butter and whisk until the mixture is fully combined.
With a rubber spatula, mix in the flour, black cocoa powder, cream of tartar and french grey salt until just combined. Fold in the remaining chocolate chips.
Spread the brownie batter into the prepared pan. Bake for 30-35 minutes or until you’re able to roll the goop that comes off of a toothpick inserted into the center into a ball between your fingers.
Sprinkle with fleur de sel and let cool in the pan completely before slicing.
Notes
*recipe can be doubled and baked in 8X8” or 9X9” pan.