Preheat oven to 350 degrees F and prepare two baking sheets with parchment paper
In a medium bowl combine the flour, oats, baking soda, baking powder, cinnamon and salt; set aside
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, granulated sugar, light brown sugar and molasses until creamy and all ingredients are fully combined
Scrape down the side and bottom of the bowl to ensure all ingredients are mixed together
Add the eggs and mix until combined
Slowly add in the dry ingredients to the butter mixture and mix until fully incorporated, scraping the sides of the bowl in between
Using a 1 ½ ounce ice cream scoop or approximately 2 inch ball, scoop the dough and place them on the prepared baking sheet spaced about three inches apart
Bake the cookies for 9-10 minutes or until the edges are light golden and the center is set (the center will slightly deflate once it is out of the oven)
Remove the cookies from the oven and allow them to cool completely on the baking sheet (they will be very soft so be sure to allow them to cool before moving them)
While the cookies are cooling, prepare the Marshmallow Buttercream
Once the cookies are completely cooled, pipe or spread the Marshmallow Buttercream onto half of the cookies and sandwich the other half together
Serve the immediately or wrap each Oatmeal Cookie Pie in plastic wrap to keep them fresh