In a medium bowl, whisk the flour, cocoa powder, baking soda, and salt together. Set aside.
In a large bowl, using a handheld or stand mixer fitted with a paddle attachment, beat the butter, brown sugar, and granulated sugar together on medium-high speed until completely combined, about 2 minutes. Add the eggs and vanilla extract, and beat on high speed for 2 full minutes. Scrape down the sides and the bottom of the bowl, then beat on high for 1 more minute.
Gradually, add the dry ingredients to the wet with the mixer on low speed until combined.
The dough will be sticky. Chill in the refrigerator for at least 1 hour for best results.
Preheat oven to 350°F. Line two cookie sheets with parchment paper or a silicone mat.
Roll or scoop the dough into golf ball-sized dough balls and place evenly spaced on the cookie sheet, about 2 to 3 inches apart.
Bake cookies for about 9 minutes. Remove from the oven and immediately add a sprinkle of flaky sea salt, if you're feeling fancy. Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Cookies stay fresh covered at room temperature for up to 1 week.
Notes
Storing leftovers. Cookies will stay fresh for up to 7 days, stored in an airtight container at room temperature.
Freezing. Baked cookies freeze well for up to 3 months. Unbaked cookie dough balls freeze well for up to 3 months. Bake frozen cookie dough balls for an extra minute, no need to thaw. To freeze the cookie dough balls, place in the fridge for 1 hour. Then, place the solid and cold dough balls into a labeled zipped-top bag. Label the bag with the month and the baking temperature and place the bag in the freezer.