In a stand mixer or in a bowl using a handheld mixer, cream the butter and sugar together until well combined. Mix in the egg until the mixture is smooth then stir in the vanilla.
Whisk in the flour, cocoa powder, baking soda, and salt until combined. The cookie dough will be sticky.
Cover the bowl in plastic wrap and chill for 1 hour or up to overnight in the fridge. Chilling the dough will help the cookies spread less and bake more evenly. For best results, don't skip this step!
Preheat the oven to 350ºF. Line a baking sheet with parchment paper and spray with cooking spray. Set aside.
Place the granulated sugar in one bowl and the confectioner's sugar in another bowl. Remove the cookies from the fridge.
Using a cookie scoop, form the dough into balls (approx. 1.5 tablespoons of dough) and roll each one into a ball with your hands.
Coat in the granulated sugar first then roll generously in the confectioner's sugar. The more powdered sugar, the better. Place the cookies on the baking sheet about 2 inches apart.
Place the baking sheet on the highest rack of the oven. Bake for 10-11 minutes until the edges are set. The cookies will be thick and fudgy in the middle. Let them cool for 5 minutes before serving. Enjoy!
Notes
Cookies will stay fresh at room temperature for 5 days and can be frozen for up to 3 months in the freezer.