Preheat the oven to 350ºF and line a baking sheet with parchment paper. Set aside.
Whisk or soft together the flour, cocoa, baking soda, and salt in a medium bowl. Set aside.
Using a stand mixer fitted with the paddle attachment or electric hand mixer, beat the butter and sugars until light and fluffy. Add the egg and vanilla, and continue beating until well combined.
Stir in dry ingredients until completely incorporated, but do not overmix.
Using a large spoon or 1-ounce cookie scoop, drop cookie dough onto the parchment lined baking sheet, leaving at least 2 inches between cookies. Gently flatten the dough ball using your palm or the bottom of a lightly floured glass.
Bake for 10-12 minutes until cookies are set on the edges, but not browned.
Remove from oven. Transfer to a wire rack after 5 minutes to cool completely.