Preheat oven to 375°F. Prepare baking sheets by lining them with parchment paper or use silicone baking mats.
In the bowl of a standing mixer, cream together the shortening and sugars until creamy and smooth. Mix in the eggs and vanilla, just until combined. Do not over-mix.
In a separate bowl, combine the flour, corn starch, baking soda, and salt. Mix well. Add the dry ingredients to the wet; mix on a low setting, just until fully incorporated. Do not over-mix.
Add chocolate chips and M&Ms, and stir in with a wooden spoon or spatula until evenly incorporated.
Using a ¼ cup cookie scoop, place cookie dough on prepared baking sheets, placing only 5 cookies per sheet to allow them room to spread out. Bake for 13-15 minutes or until edges are golden brown. Do not over-bake.
Take out of the oven and immediately top with additional M&Ms. Allow to rest for 2 to 3 minutes, then move to a cooling rack to completely cool.
Allow your pans to come to room temperature before placing more cookie dough on them. For a quicker method, run your pan under cool water for 20 to 30 seconds, dry, and continue with your baking.