Preheat the oven to 400 F. Add the butter and brown sugar to a bowl or stand mixer. Mix for 2-3 minutes until fully combined and creamy. Add peanut butter and mix another minute. Add eggs and vanilla extract. Mix until just combined.
Add flour, cornstarch, baking soda, and salt. Mix until just combined. Add peanut butter m&ms and mix until they're just distributed through the dough.
Using a kitchen scale, scoop out 6 ounce of dough and form into a rough ball. Place up to 6 dough balls on a lined or lightly greased cookie sheet.
Bake for 9-11 minutes until cookies are just starting to brown. Let the cookies cool on the baking sheet for 5 minutes, then transfer to a cooling rack to cool completely.
Keep any extra cookies in a sealable container. Enjoy!!