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. Scrape down the bowl as needed to make sure it's mixed evenly.
Bake until edges are crisp and the tops just barely appear set (no longer wet), about 9 minutes. Baking times may vary based on your oven and cookie size!
Immediately sprinkle with coarse sugar or salt while cookies are still hot. If they're misshapen, use a spatula to push them back into a circle. Let them 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 (if they last that long!).
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Notes
Measure flour correctly! Several readers have had issues with the cookies not spreading, being too thick, or needing to add a little water to the dough to get it to come together. This is most likely an issue with adding TOO MUCH FLOUR because of not measuring it correctly. If possible, use a kitchen scale, or use the "spoon and level" method when using measuring cups (more info through that link).
If using Dutch-process cocoa powder, change 1 tsp baking soda to 1 tsp baking powder. (Traditional unsweetened natural cocoa powder is acidic and will react with the baking soda, but Dutch-process cocoa powder is not acidic and so you will need to use baking powder instead to get the same rise.)
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