Preheat the oven to 350°F (175°C).

In a mixing bowl, whisk together the flour, cocoa powder, baking soda, and salt.

In another bowl, cream the softened butter and sugar until light and fluffy.

Add the egg and vanilla extract to the butter mixture and mix until well combined.

Gradually add the dry ingredients to the wet mixture, mixing until a dough forms.

Scoop tablespoon-sized portions of dough and roll them into balls.

Place the dough balls on a baking sheet lined with parchment paper, spacing them about 2 inches apart.

Flatten each ball slightly with the bottom of a glass.

Bake in the preheated oven for 8-10 minutes, or until the edges are set.

Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.