Preheat oven to 350 °F (177 °C) and line baking sheets with silicone baking mats or parchment paper. Set aside.
In the bowl of a stand mixer, cream together the butter and sugars until light and fluffy, about 2 minutes. Then add the molasses, vanilla, and eggs. Beat until well incorporated.
In a medium bowl, combine flour, baking soda, salt, and cinnamon. Slowly add the flour mixture to the wet mixture and beat until all is combined. Stir in the oats.
Drop the dough by the tablespoon onto the prepared baking sheets about 1 inch apart.
Bake for 11-12 minutes or until the edges start to turn brown. Don't overcook them! They will look moist when you take them out of the oven but that's what you want for a soft cookie!
Let sit on baking sheet for 5 minutes before transferring to a wire cooling rack to cool completely.
While cookies are cooling, make your filling.
In another bowl of your stand mixer, cream together the marshmallow fluff and butter for 2-3 minutes. Slowly add the powdered sugar and vanilla. Whip until light and fluffy then set aside.
To assemble the creme pies, use the cookie scoop you used for the cookie (cleaned, of course) and scoop marshmallow fluff on the bottom side of one cookie, then take another cookie and sandwich it together.
Continue until all the cookies have been made into sandwiches.
Store in airtight container up to a week, I suggest putting it in the fridge and then setting them out half an hour prior to eating so the fluff will soften up.