In a stand mixer fitted with a paddle attachment, cream together butter and brown sugar. Add peanut butter and mix until incorporated.
Add eggs and vanilla. Mix until combined.
Sift together flour, baking soda and salt. Add dry mixture and beat until just incorporated. Do not over mix but be sure there is no sugar stuck on bottom of bowl.
Cover bowl with plastic wrap and chill for 1 hour.
Using a small cookie scoop (about 2 tsps in size) scoop dough on cookie sheets. Be sure not to place more than 6 cookies per sheet, these cookies will spread. Using your thumb or index finger press a "hole" or indent just off-center in cookie dough.
Fill a piping bag (or ziplock bag with the corner cut off) with 1 cup marshmallow fluff. Pipe about 1 tsp of fluff in each cookie.
Top cookie with another scoop of dough. (After this step I like to form them a little bit, just closing where the fluff sticks out)
Bake at 375 degrees for 8-9 minutes. Let cool on baking sheet for 5-6 minutes before transferring them to cooling rack.