Preheat oven to 350°. Prepare baking sheets lined with parchment paper or silicone baking mats and set aside.
Using mixer, beat butter and sugar until it’s very light and fluffy.
Beat in the eggs on medium-low speed one at a time until well combined.
In another medium bowl, whisk together the dry ingredients, flour, cocoa powder, baking soda, cornstarch and salt. Add the flour mixture to the butter mixture and combine on low until just combined.
Add the m&ms, chocolate chips and chunks and combine on low until evenly distributed, being careful not to overmix.
Using a large cookie scoop, scoop cookies onto a sheet pan lined with parchment paper 2 inches apart. Add extra m&m's, chips/chunks on top if desired!
Bake cookies for 12-15 minutes, until the edges look set and the top is mostly dry. Cool for 10 minutes on the sheet pan.