In a large mixing bowl of a stand mixer, cream together the butter, sugar, and brown sugar. Scrape the bottom and sides of the bowl occasionally with a rubber spatula.
Add the egg, molasses, vanilla, salt, quick oats, flour, baking soda, and cinnamon and incorporate. Scrape the bottom and sides of the bowl again with a rubber spatula so everything gets mixed well.
Use an ice cream scoop to scoop even balls onto half sheet pans lined with parchment paper. Space them evenly apart so they have enough room to spread and not touch each other.
Place into a preheated 375 degree F oven for 14 minutes. Turn your baking pans once during the baking process.
Let the cookies cool completely before placing frosting on cookies.
Cream filling
In a large mixing bowl of a stand mixer using a beater attachment, beat together the ingredients. Add heavy cream a little at a time and adjust as needed for your desired thickness. Scrape the bottom and sides of the bowl with a rubber spatula.
Scoop the cream filling into disposable pastry bags or a large zip-style bag. Cut the tip off the pastry bag and squeeze some cream filling into the center of one cookie. Press another cookie on top and sandwich together.
Place cookies into individual baggies or an airtight container and enjoy immediately, Or refrigerate any remaining cookies. They will last up to 1 week in the fridge.
Notes
Oatmeal Cream Pie Filling:Should you want a slightly less sweet filling option, try adding 1 cup of marshmallow fluff to the filling and 1 cup less powdered sugar.