Preheat oven to 350°F. Line 2 baking sheets with parchment paper (don't skip this step, it helps keep the cookies from spreading).
Using a hand mixer or stand mixer fitted with the paddle attachment, beat butter and sugars for several minutes, until light and fluffy. Add egg and vanilla, beat until combined.
In a separate bowl whisk flour, cocoa powder, baking soda and salt.
Add dry ingredients to butter mixture and beat until just combined. Use a rubber spatula to stir in chocolate chips and ½ cup M&M's. At this point, I like to let the dough rest at cool room temp for 30 minutes before baking (this helps with the texture of the cookies). If you prefer your cookies thick, you can transfer the dough to the refrigerator for 30 minutes.
Scoop the dough into 2-tablespoon portions, roll into balls and place a few inches apart on parchment lined baking sheets. Lightly press the remaining ¼ cup M&M's onto the tops of the dough mounds.
Bake for 9-11* minutes, rotating the baking sheets from top to bottom and front to back. Bake until the edges are beginning to set, but the rest of the cookie is still slightly underdone. Note: They should be so soft that you should not be able to pick them up immediately after removing from the oven. They will set as the cool.
Place the baking sheet on wire rack to cool completely. Repeat with remaining dough, allowing the baking sheets to cool completely between batches.