12ouncesbittersweet chocolateuse the best quality chocolate you can, cut into roughly 1/2-inch chunks with a serrated knife.
flake saltfor finishing
In a medium bowl, cream the butter and sugars together with an electric mixer on medium speed, until light and color and texture, 5 minutes. Scrape down the sides of the bowl with a rubber spatula as needed. Add the egg and vanilla and mix well to combine. Again, stop the machine and scrape down the bowl with a spatula.
In a medium bowl, whisk the flour, baking soda, and salt to combine.
With the mixer on low, slowly add the flour mixture. Mix until streaks of flour still run throughout. Add the chocolate and mix until everything just comes together. Finish the dough by hand, taking care to scrape the sides and the bottom of the bowl to make sure everything is evenly distributed.
At this point, it is best to refrigerate the dough for 24 hours. Resting the dough intensifies the deep toffee flavors, and the texture of the baked cookie improves.
When ready to bake, preheat the oven to 360ºF. Line two baking sheets with parchment paper. Scoop the dough onto the baking sheets. Top the cookies with a pinch of flake salt just before baking.
Bake for 10 to 12 minutes. Rotate the sheets halfway through if they appear to be baking unevenly. The cookies should be lightly golden on he outside but still look quite gooey on the inside. Let the cookies cool on the baking sheets for at least 5 minutes. Transfer the cookies to a wire rack to finish cooling.
These are best eaten the day of baking but will keep, if well sealed, for up to 2 days.