• Preheat the oven to 350 degrees F (177 degrees C).  Line baking sheets with parchment paper.
• Using a stand mixer, cream the butter, dark brown sugar, salt, cinnamon, nutmeg, egg, and vanilla together.  Once it’s light and fluffy, scrape the sides and bottom of the bowl with a spatula.  Add the baking soda, GF flour, and oats and mix on low until combined.  Turn the mixer speed up to medium-high and mix for a minute or two until very well mixed.
• Scoop 8-12 dough balls onto each parchment lined baking sheet, spreading them out evenly so that they won’t ooze into each other while baking.  Bake in the preheated oven for about 8-11 minutes or until lightly golden brown.  Remove from the oven, and let them cool completely at room temperature.

For The Filling:

• In a large bowl using an electric hand mixer (or the cleaned stand mixer), beat the butter, salt, and vanilla until light and fluffy.  Add the powdered sugar in three separate increments (½ cup at a time) and mix well in between each one.  Scrape the sides and bottom of the bowl with a spatula.  Mix on high for a couple minutes until an airy and very fluffy frosting forms (should look white, instead of a cream/butter color, from the fluffiness).  Finally, add the marshmallow fluff and mix until well incorporated.  If you live in a hot place or it’s very warm in your kitchen, place the filling in a cooler place or the fridge until the cookies are completely cooled.

Assembly:

• Spread or pipe some filling on the bottom side of one cooled cookie and place another cookie on top, sandwiching them together.  Repeat these steps with the remaining cookies.  Enjoy!  You can store them at room temperature in an airtight container for a couple days then place in the fridge if there are any left.  They also freeze well!