Preheat your oven to 350°F (175°C).

In a mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.

Beat in the eggs one at a time, then mix in the vanilla extract.

In a separate bowl, whisk together the flour, baking soda, cinnamon, and salt.

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

Add the oats to the dough and mix until evenly incorporated.

Drop tablespoon-sized amounts of dough onto lined baking sheets, spacing them about 2 inches apart.

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

Let the cookies cool slightly on the baking sheet before transferring them to a wire rack.

Once cooled, mix marshmallow fluff, powdered sugar, and vanilla for the filling, then spread it between two cookies.