Preheat oven to 350 degrees and line 3 baking sheets with parchment paper.
In a medium bowl, cream butter, sugar, and brown sugar together with an electric mixer on medium speed four about 2 minutes, until smooth and light colored.
Add egg and vanilla extract and continue mixing for another minute.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and sea salt.
Add flour mixture, half at a time and mix with a rubber spatula until just combined.
Using a 3 TBS cookie scoop, scoop dough and place 2 inches apart on the cookie sheet.
Bake for 10 minutes, rotating the pan halfway through.
Allow cookies to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
Store cookies in an airtight container for up to a week or freeze for up to 3 months.
Notes
Make sure to read the cookie baking tips to get answers to common questions about baking cookies.