Preheat your oven to 350°F and line a baking sheet with parchment paper. Set aside.
In a large mixing bowl combine flour, cocoa powder, baking soda and salt. Stir gently to combine. Set aside.
In another large mixing bowl cream together softened butter, granulated sugar and brown sugar until light and fluffy (about 2min). Add eggs and vanilla and mix until thouroughly combined (about 2min).
Gradually add the flour mixture to the butter mixture and stir until just combined.
Add M&M's and mix until well incorporated.
Drop by well-rounded tablespoons* on to a parchment lined baking sheet, placing cookies about 2 inches apart.*I used a #40 disher (¾oz).
Bake cookies for about 9-11 minutes or until the centers of the cookies are set.
Allow cookies to cool on the baking sheet for 2-3 mintues before removing to a wire rack to cool completely.
Notes
Cookies will last 3-4 days kept in an air-tight container or zip top bag, at room temperature.