Preheat the oven to 350°F and line two large baking sheets with parchment paper.

In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, brown sugar, and granulated sugar until light and fluffy, about 2 minutes. Beat in the egg and vanilla extract, scraping down the sides of the bowl as needed.

In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt, then mix in the chocolate chips.

Gradually add the dry ingredients to the stand mixer, mixing on low speed after each addition. After the final addition, mix until well combined.

Use a 2-tablespoon cookie scoop to scoop the dough onto the prepared baking sheets, leaving 2 inches between cookies. Press more chocolate chips into the tops of the cookies, then sprinkle with flaky sea salt.

Bake, one sheet at a time, for 9 to 10 minutes, or until the edges of the cookies are very lightly set. The cookies will still seem soft and underdone at this point, but they will set up more as they cool. Allow to cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.