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, peanut butter and sugar until fluffy.
Add in the egg and vanilla extract and mix until smooth. Add in the flour mixture and mix until combined. Evenly press the cookie dough into a greased square baking dish and bake for 20-25 minutes.