In the bowl of a stand mixer, cream together the butter and peanut butter until smooth.
Add the brown sugar & white sugar, creaming together until fluffy.
Add in the eggs and vanilla, beating well.
In a separate bowl, mix the baking soda, salt, cornstarch, and flour, until combined.
Then add the dry ingredient mixture to the wet ingredients.
Stir in the oats, M&M's, chocolate chips, and butterscotch until just combined.
Cover and refrigerate dough for at least 2 hours, up to overnight. (we refrigerate overnight)
Preheat oven to 350 degrees F. Line 2 baking sheets with parchment or silicone liners.
Drop heaping 1/4 cup-sized balls of dough onto the baking sheets, about 2-3" apart.
Bake for approximately 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 baking sheet for at least 5 minutes then transfer to racks.