Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.
In a large bowl, cream 1 cup butter, brown sugar, and granulated sugar until light and fluffy.
Beat in the egg, vanilla extract, and molasses until well combined.
In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon.
Gradually incorporate the dry ingredients into the butter mixture, then stir in the quick oats.
Scoop 2-tablespoon portions of dough onto the baking sheets, spaced 2 inches apart.
Bake for 10-12 minutes or until edges are lightly browned but centers are still soft.
Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
For the filling, beat 1/2 cup butter, powdered sugar, marshmallow fluff, and vanilla until smooth and aerated.
Spread or pipe a generous layer of filling onto the flat side of half of the cooled cookies.
Top with the remaining cookies to create sandwich pies.