Preheat oven to 350 F.
Line baking sheet with parchment paper and set aside.
Cream butter and sugars together until smooth.
Add pudding, eggs, additional egg yolk and vanilla. Beat for about one minute on medium speed.
In a separate bowl, combine flour, cocoa powder, baking soda, and salt.
Add dry ingredients to the wet ingredients a ½ cup at a time and mix until combined.
Fold in chocolate chips, marshmallows and walnuts.
Use a medium cookie scoop (1½ tbsp) to place scoops onto prepared baking sheet about two inches apart.
Bake for 10-13 minutes.
Cool on pan for a couple of minutes before transferring to a cooling rack.