Preheat oven to 350°F. Line two baking sheets with parchment paper; set aside.
In the bowl of an electric mixer on medium speed, beat together butter, peanut butter, and sugars until creamy.
Add eggs, vanilla, and baking soda, beating well to combine after each addition.
Reduce mixer speed to low; add cocoa, then flour. Beat to combine. Stir in chips until evenly distributed.
Using a small cookie scoop , drop dough onto prepared baking sheets. Bake for 8-9 minutes, until set. Allow to cool completely before filling.
Prepare the filling: In the bowl of an electric mixer fitted with the whisk attachment, on medium speed, beat together butter, peanut butter, and marshmallow fluff until smooth. Add sugar; beat until fluffy.
Assembly: Spoon or pipe the filling onto the flat side of a cookie; top with a second cookie.
Notes
I got 24 sandwich cookies. I used a #18 scoop - about a 2 tabslesoon sized scoop. These last in the freezer, let thaw to room temperature before serving