Preheat oven to 350 degrees F
Line baking sheets with tinfoil and set aside.
Combine butter, brown sugar, egg, vanilla, baking soda, and salt and mix until light and fluffy, 2-3 minutes (hand or stand mixer)
Add flour and oats and mix.
Scoop the cookie dough and form into balls and place several inches apart on baking sheets.
Bake for 8-10 minutes — DON’T overbake these cookies! Let cookies cool.
For filling, with an electric mixer whip together butter, powdered sugar, marshmallow creme, vanilla and heavy cream.
Spread filling on bottom of a cookie and top with another cookie. Repeat.