1Combine softened butter, eggs, brown sugar, granulated sugar, 2 teaspoons of vanilla extract, and molasses in a bowl and mix well.
2Add cinnamon, baking soda, and all-purpose flour to the mixture and mix on low speed until almost all incorporated.
3Add quick cooking oats and finish incorporating the mixture with a silicone spatula instead of the mixer.
4Use a medium cookie scoop to drop the cookie dough onto cookie sheets.
5Bake at 350° for 9 to 11 minutes, or until the edges start to form.
6Let the cookies sit on the cookie sheet for 2 to 5 minutes before removing them to a wire cooling rack.
7Ensure cookies are fully cooled before preparing the filling.
8For the filling, mix 3/4 cup of butter with a mixer.
9Add half of a 454g container of marshmallow fluff, 2 teaspoons of vanilla extract, and 1 tablespoon of milk to the butter and mix again.
10Gradually add approximately 2 cups of icing sugar, mixing after each addition, until the taste and consistency are content. Add an additional tablespoon of milk if the consistency is too thick.
11Fill the cooled oatmeal cookies by adding the desired amount of filling to the bottom side of one cookie (the side that touched the cookie sheet) and then topping with another cookie, also bottom-side down.