Preheat your oven to 350F°. Beat butter, shortening and peanut butter until smooth. You might have a hard time getting some of the shortening to blend well, don't worry as long as you get the chunks down to pea size or smaller for now. Add in both sugars and beat well. Add eggs and vanilla, mixing well. Slowly add in your remaining dry ingredients. Remember to scrape the sides of your bowl. Mix well until you have a nice smooth dough, then add in your m&ms.
Using a small or medium sized cookie dough scoop, put about 8 scoops on your cookie tray. With a fork, push down on each cookie horizontally and then vertically so you have made crisscrossed impressions. Bake 9 minutes for small cookies and 10-11 minutes for medium sized cookies. The larger your cookies are the more they will spread and the longer they will take to bake. I started with large cookies and switched to small cookies after I saw how much they spread while cooking. Medium sized might be the perfect size but I never tried my medium scoop.
Peanut Butter M&M Cookies - https://www.bakedbyrachel.com/peanut-butter-mm-cookies/