Preheat oven to 350 degrees and line 2 baking sheets with parchment paper or silicon mats.

In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon. Mix in the oats until well combined. Set aside.

Add the melted butter, granulated sugar, and brown sugar to the bowl of a stand mixer. Mix on medium-low speed until well combined. Add the vanilla extract and eggs and mix until light and airy, about 1 minute on medium speed - scrape down the bowl as necessary.

Add the dry ingredient to the wet ingredients and mix on medium-low speed until completely combined. Scoop the dough into 1 tablespoon balls and drop on the prepared cookie sheets, baking 10 per sheet. Bake at 350 degrees for 8-10 minutes or until done. Bake in batches as needed. Cool on the baking sheet for 5 minutes then cool on a wire rack until completely cooled.

To make the frosting, add the butter, powdered sugar, and marshmallow fluff to the bowl of a stand mixer and mix on medium-low speed. Add the cream and vanilla extract and mix until combined. Beat on medium speed for 3 minutes, until light and fluffy.

Spread the frosting on one cookie then top with a second. Repeat for all cookies. Serve immediately or store at room temperature in an airtight container until ready to serve. Cookies will stay fresh for 3-4 days.