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 both brown and granulated sugar until fluffy.
Add in the egg and vanilla extract, and mix until smooth. Add in the flour mixture, mix until it is fully combined. Fold in the M&Ms. 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.