Preheat Oven to 350 F
In a stand mixer with the paddle attachment cream the eggs and butter on medium speed for about 1-2 minutes, until fluffy.
Add the sugars and vanilla and mix well for about 1 minute on medium speed.
Add salt, oat flour, all-purpose flour, baking soda, xanthan gum and mix well on medium for about a minute.
Add oats and mix on medium for another minute.
Add chocolate chips last and let stir for a moment until chocolate chips are well combined.
Bake about 14-17 minutes.
Allow cookies to cool overnight or for at least 4 hours before adding the filling
Make the crème filling by beating in stand mixer the butter and shortening for 2 minutes.
Add the marshmallow fluff and vanilla.
Add the powder sugar 1 cup at a time.
Add the dairy free milk, scrape the sides of the bowl and beat on high for 3 minutes until the frosting is thick and fluffy.
Fill a pastry bag with the frosting (or you could frost with a knife but the pastry bag makes it look pretty) and frost the bottom of one cookie and then place another cookie on top to make the crème pie.