Preheat the oven to 350 degrees. Arrange three racks evenly spaced in the oven (see note).
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter on medium speed for 3 minutes, until creamy. Add the granulated sugar and brown sugar and beat on medium speed for 2 to 3 minutes, until light and fluffy. Scrape down the bowl with a rubber spatula. Add the egg, vanilla, and 2 tablespoons of warm water and mix on low speed just to combine. In a medium bowl, whisk together the flour, baking soda, and kosher salt. With the mixer on low, slowly add the flour mixture, then the chocolate (including the fine chocolate dust) to the batter until combined. Mix well with a rubber spatula.
With a 2¼-inch standard ice cream scoop (or ¹⁄₃ cup measure), make 12 rounded scoops of dough and place them on a sheet pan. Freeze the dough for exactly 15 minutes, then arrange 4 balls of dough—spaced wide apart—on each of three sheet pans lined with parchment paper. Bake for 10 minutes, until the cookies are slightly puffed in the center. Remove the pans from the oven and bang them on the stove top, until the center of the cookies deflate. Bake for 3 minutes, then bang the pans again, repeating baking and banging every 3 minutes, for 18 to 20 minutes total, until the edges of the cookies are golden brown. (The centers will be lighter and not fully cooked.) Rotate the sheet pans in the oven so the cookies bake evenly. Sprinkle the cookies with fleur de sel and cool completely on the pans.
If your oven has only two racks or you have only two sheet pans, bake these in two batches but don’t freeze any batch of dough for more than 15 minutes.
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