Preheat oven to 350°F. Line a sheet pan with parchment paper.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar on medium high speed until light and creamy, about 4 minutes. Add the eggs one at a time, mixing after each. Add the vanilla.
In a separate bowl, sift together the flour, cocoa powder, baking soda, and salt. Add to the mixer and mix until just combined. Scoop by the rounded tablespoon onto the prepared sheet pan and flatten slightly with your hand. Sprinkle the tops with fleur de sel.
Bake until edges are crisp, about 9 minutes. Let cool on the pan for a few minutes before moving to a wire rack to cool completely.
Store the cookies in an airtight container at room temperature for several days.