Preheat oven to 350F. In a mixer, cream the butter and sugar for 5 to 10 minutes. Add the eggs one at a time, until well-combined.
Add vanilla and peanut butter and mix well. Next, add the dry ingredients, slowly, mixing well after each addition.
Scoop 3/4 ounce spoonfuls onto a baking sheet. Bake for 10 to 12 minutes. Press cookies down with a spatula to flatten when they come out of the oven.
Filling: Combine peanut butter, butter, and sugar, and cream until smooth. Put the peanut butter mixture and marshmallow fluff in separate pastry bags. Pipe the peanut butter mixture on one cookie, then the marshmallow fluff on another. Sandwich together.