Preheat the oven to 350°F (175°C).

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

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

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 old-fashioned oats until evenly distributed throughout the dough.

Drop the dough by rounded tablespoonfuls onto a parchment-lined baking sheet, spacing them about 2 inches apart.

Bake for 10 minutes at 350°F (175°C), or until the edges are golden and the centers look slightly underbaked.

Allow the cookies to cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.

Meanwhile, in a small bowl, combine the marshmallow fluff, powdered sugar, and vanilla extract (for filling) until smooth.

Once the cookies are completely cool, spread the filling on the bottom of one cookie and sandwich it with another to form the oatmeal cream pie.