Preheat the oven to 350F degrees and line cookie sheets with parchment paper or baking mats.
In a medium bowl whisk together the flour, cinnamon, baking soda, and salt.
In a separate large bowl, beat together the melted butter, brown sugar and white sugar.
Beat in the molasses, vanilla extract and eggs.
With the mixer on low speed, mix the dry ingredients into the butter mixture. Then mix in the oats. The dough will be sticky.
Spoon the dough into balls about 1 tablespoon of batter each (a little smaller than a golf ball) and place 2 inches apart on the lined cookie sheets. I highly recommend using a cookie scoop for uniform cookies.
Bake 1 sheet at a time in the preheated oven for about 10 minutes, or until the tops are set. The cookies will spread as they bake. If you feel they're too thin, chill the dough in the fridge for 30 minutes. If the cookies aren't flattening as much as you'd like, flatten the dough balls slightly before baking.
Cool the cookies on the baking sheet for about 10 minutes, then transfer to a wire rack to continue cooling.
In a large bowl, beat the butter until soft.
Stir in 1 cup powdered sugar and the salt.
Beat in the marshmallow fluff. Stir 1 tablespoon of cream (or milk), adding in a little extra powdered sugar or a little more cream as needed.
Frost the bottom of 1 cooled cookie, and place a second cookie on top.
Store cookies in an airtight container at room temperature for up to 3 days, or in the fridge for up to 5 days. Baked and unfrosted oatmeal cookies can be frozen for up to 6 months.