Preheat your oven to 350°F (175°C). Line your baking sheets with parchment paper.
In a medium bowl, whisk together flour, cocoa powder, baking soda, and sea salt.
In a large bowl, cream the softened butter with brown sugar until light and fluffy.
Add the egg and vanilla extract, mixing just until combined.
Gradually mix in the dry ingredients until no flour is visible. Avoid overmixing.
Scoop out tablespoon-sized portions of dough. Roll into balls by hand.
Coat dough balls by rolling them in granulated sugar until fully covered.
Place dough balls on lined baking sheets about 2 inches apart.
Bake for 10–12 minutes until edges are set but centers are still soft.
Cool cookies on baking sheet for 5 minutes, then transfer to wire rack to cool completely.
Notes
Don't chill the dough before baking to achieve the classic crackle-top texture. Cookies store well at room temperature for 4–5 days or can be frozen up to 3 months.