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.
Bake until edges are crisp, about 9 minutes. Immediately sprinkle with coarse sugar or salt. Let cool on the pan for a few minutes, then transfer to a wire rack to cool completely.
Repeat with remaining cookie dough.
Store the cookies in an airtight container at room temperature for several days.