Preheat the oven to 350˚F and line two baking sheets with parchment paper. 

In a large mixing bowl, whisk together the melted butter, molasses, granulated sugar and brown sugar. Add in the eggs and vanilla extract and whisk until smooth. 

In a separate medium bowl, whisk together the all purpose flour, oats, baking soda, salt and cinnamon. 

Add the dry to the wet ingredients in two batches, stirring just until fully combined and no streaks of flour remain. 

Using a size 50 cookie dough scoop (holds ~1 ¼-1 ½ tablespoons of dough), scoop the dough onto the baking sheet, then use your hand to gently flatten the ball into a disc. Leave about 1 ½ inches between each cookie. 

Bake for 9-11 minutes, until the bottoms and edges are golden brown. Remove from the oven and let cool for a minute, then transfer to a wire cooling rack to cool completely before filling. 

To make the filling, add the butter to a mixing bowl and beat on medium-high speed for 2 minutes, until light and fluffy. Add the powdered sugar and continue mixing for 1 minute. Add the marshmallow fluff and salt and beat until well combined. It should be a fairly thick, stiff mixture. If it doesn't seem thick enough, add additional powdered sugar 2 tablespoons at a time. 

Using a piping bag with a round tip, pipe about 1 tablespoon of filling onto the bottom of half of the cookies, then sandwich with another cookie. Enjoy!