Preheat Oven to 350 Degrees
In a large bowl, sift together: flour, oats, baking soda, baking powder, salt, and cinnamon.
In an electric mixer, combine the butter and brown sugar on medium speed until light and fluffy.
Mix in the vanilla and egg on low speed.
Keep the mixer on low speed and slowly add in the dry ingredients. Increase the speed gradually until combined.
Add heaping tablespoon-sized drops of dough onto a baking sheet. Keep the cookies about one and a half inches apart.
Bake 7-8 minutes. They'll be lightly brown around the edges and soft in the center. They'll continue to cook once you take them out of the oven.
Use a spatula to transfer the cookies to a cooling rack.
Add the butter, vanilla, and marshmallow cream to the electric mixer and combine on medium speed. (Make sure there are no chunks of butter.)
Slowly add the powdered sugar and mix on very low speed.
Once the powdered sugar is combined, whip the filling on high-speed for twenty seconds.
Flip the oatmeal cookies upside down with the flat surface facing up, and frost it with about two tablespoons. (I usually add one tablespoon to each cookie bottom) Add another cookie to make a sandwich.
Frost the top of the oatmeal cookies for the open-faced style!