Preheat oven to 350°F. Line two 9x13 baking dishes with parchment paper and spray with nonstick spray.
In a large bowl, beat 2 ½ sticks of butter, brown sugar, and sugar until light and creamy.
Add vanilla and eggs, beat until combined.
In another bowl, mix flour, cinnamon, salt, and baking soda. Add to wet mixture and beat until combined.
Stir in both types of oats and beat again.
Divide dough between the two pans. Spray your hands (or a spoon) with nonstick spray and press dough evenly into each pan.
Bake for about 13 minutes, or until golden brown. Let cool completely.
While bars cool, make the filling: beat 1 ½ sticks of butter until creamy. Add powdered sugar, beat again. Add marshmallow fluff and vanilla, beat until smooth.
Once bars are cool, lift one out of the pan and place on a tray. Spread the cream filling evenly on top.
Lift the second bar out and carefully flip it on top of the cream layer. Press down gently.
Freeze for 1 hour to set.
Slice into bars and serve!