In the bowl of a stand mixer, cream together the butter and peanut butter until smooth. Add in the brown sugar & sugar, creaming together until combined. Add in the eggs and vanilla, beating well. Then add in the baking soda, cornstarch and flour, mixing until combined. Mix in the oats, M&M's and chocolate chips last until just combined.

Cover and refrigerate dough for at LEAST 1 hour, up to overnight.

Preheat oven to 350 degrees F. Line 2 baking sheets with parchment or silicone liners, or spray lightly with cooking spray. Drop heaping 1/4 cup-sized balls of dough onto the baking sheets, about 2-3" apart. Bake for approx. 12-14 minutes, rotating pans halfway through baking time to ensure even cooking. Cookies may appear slightly undone in the center; this is normal and they'll continue to set as they cool. Cool on the racks. Serve warm or at room temperature.