Using a hand mixer or stand mixer fitted with the paddle attachment, beat butter and sugars for 1-2 minutes. Add egg and vanilla, beating until combined, and scraping the sides and bottom of the bowl as necessary..
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 your kitchen is warm you can place the dough in the refrigerator for 30 minutes.
Preheat oven to 350°F. Line 2 baking sheets with parchment paper (don't skip this step, it helps keep the cookies from spreading).
Scoop the dough into 2-tablespoon portions and form into mounds that are higher rather than wider. Place the dough mounds a few inches apart on parchment lined baking sheets. Lightly press the remaining ¼ cup M&M's onto the tops of the dough (this makes for a prettier cookie).
Bake for 9-11* minutes, 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 off the baking sheet. They will set as the cool.
Place the baking sheets on wire rack to cool completely. See note about shaping the cookies while they are still warm.
Notes
*If you chilled your dough in the refrigerator, you will likely need to add a few minutes to the bake time. How to get perfectly round cookies:Option 1: Immediately after the cookies come out of the oven place a round glass, bowl, or cookie cutter (larger than the cookie) upside down over the top of the cookie (see photo below). Gently but quickly swirl the bowl in a circular motion. The edges of the cookie will bump against the inside of the bowl creating a perfectly round shape. Note: This only works when the cookies are hot, straight from the oven, and still on the warm baking sheet.Option 2: Immediately after the cookies come out the oven use the back of a spoon to gently push the edges inward to create clean edges and a round shape.Serving and storage:These cookies are best the day they baked. They also freeze well. If desired, you can heat them briefly in the microwave to make them soft and fudgy.