Preheat your oven to 350°F (175°C). Prepare a baking sheet by lining it with parchment paper.

In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar together until the mixture is fluffy and well-combined.

Crack in the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract until smooth.

In another bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, and salt.

Gradually add the dry mixture to the wet ingredients, mixing until just combined.

Fold in the rolled oats until evenly distributed throughout the dough.

Using a cookie scoop, portion out the dough onto the prepared baking sheet, leaving space between each dollop.

Bake for 10-12 minutes or until the edges are lightly golden.

Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.

In a separate bowl, mix the marshmallow fluff and powdered sugar until well combined.

Once the oatmeal cookies are completely cool, spread a generous amount of marshmallow filling on the flat side of one cookie, then sandwich it with another cookie.