Grease a baking sheet or line with parchment paper and preheat oven to 350℉.

In a medium-sized bowl, add the flour, cocoa powder, baking soda and salt. Whisk until mixed well. Set aside.

In a large bowl, add the butter. Microwave for 25-30 seconds, until butter is soft and just barely starting to melt. Add both sugars to the bowl.

Using a mixer, cream together the sugar and butter by beating on low for about 1-2 minutes, or until well incorporated.

Add the egg and vanilla and beat until just combined. (Don't overbeat.)

Add the bowl of dry ingredients to the wet mix. Beat for about 2 more minutes or until dry ingredients are well incorporated. (It should form a ball and be easy enough to roll in your hand without sticking to it too much.)

Shape dough into balls and place on a baking sheet about an inch or so apart. (Two level tablespoons of dough each makes about 15 cookies.)

Bake at 350℉ for 10 minutes. Careful not to overcook. Remove from oven and let stand on the baking sheet for at least 5 minutes. Move to a cooling rack and once completely cooled, place in an airtight container and store at room temperature for up to 5 days.