Preheat the oven to 350 F. In a medium bowl, whisk together flour, cocoa powder, salt, and baking soda. In a large bowl, mix together butter and sugar until fluffy. Add in egg and vanilla extract and mix until it is smooth.
Add in the flour mixture and mix until it is fully combined. Place powdered sugar into a shallow dish. Roll the cookie dough into balls and roll in the powdered sugar. Place cookies onto a large ungreased baking sheet at least 2-inches apart.
Bake for 10-12 minutes. Remove from the oven and top each one with a chocolate kiss.