Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper.

In a bowl, whisk together flour, baking soda, cinnamon, and salt until evenly mixed.

In a separate bowl, cream together butter, brown sugar, and granulated sugar until fluffy — about 2–3 minutes with a mixer. Mix in vanilla, then add eggs one at a time, beating until combined.

Gradually add the dry ingredients to the wet ingredients until just combined, then stir in oats with a spatula. The dough will be thick.

Drop dough by tablespoonfuls onto prepared baking sheets, spacing about 2 inches apart. Slightly flatten each mound with your fingers — they don’t spread much.

Bake for 10–12 minutes or until the edges are golden and centers look set. Rotate pans halfway if your oven is uneven. Let cookies cool on the baking sheet for a few minutes, then transfer to a wire rack to cool completely.

To make the filling, beat together marshmallow fluff, powdered sugar, and vanilla until smooth and spreadable. If it’s too stiff, add a tiny splash of milk.

Sandwich a spoonful of filling between two cooled cookies. Press gently so the filling spreads to the edges. Enjoy right away or chill to set the filling.