Preheat your oven to 350 degrees.
Cream together your butter, peanut butter, sugar and brown sugar until well mixed.
Add in your egg yolk and vanilla and mix well.
Add in your dry ingredients, your baking soda, salt and flour and stir everything to combine.
Mix in your chocolate chips and m&ms until fully incorporated.
Bake for about 10 minutes until cookies are set and lightly golden.
Allow cookie to rest for about 10 minutes on baking pan before moving to a cooling rack.