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 sugar until fluffy.
Add in egg and vanilla extract, and mix until it is smooth. Add in the flour mixture and mix until the flour is fully combined. Fold in the m&ms.
Roll the cookie dough into balls, and place them onto a large ungreased baking sheet. Bake for 10-12 minutes.