In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy—about 2–3 minutes using a hand or stand mixer.
Mix in the egg and vanilla extract, beating until fully combined.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.
Preheat oven to 350°F (175°C) and line baking sheets with parchment paper. Scoop out tablespoon-sized balls of dough and roll each in granulated sugar. Place 2 inches apart on the baking sheet. Bake for 9–10 minutes until the edges are set. Let cool on the baking sheet for 5 minutes before transferring to a wire rack.