Preheat the oven to 350 F. In a medium bowl, whisk together flour, cocoa powder, salt, and baking soda.
In a large bowl, combine butter with granulated sugar and brown sugar until fluffy. Add in egg and vanilla extract, and mix until smooth. Add in the flour mixture, and mix until a thick dough forms.
Place the powdered sugar into a shallow dish. Roll the cookie dough into balls, and place them onto a large ungreased baking sheet at least 2-inches apart. Bake for 10-12 minutes.