Preheat your oven to 350°F (175°C).

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

Add the egg and vanilla extract, mixing until fully incorporated.

In another bowl, sift together the flour, baking soda, cinnamon, and salt.

Gradually add the dry ingredients to the wet mixture, stirring in the oats until just combined.

Drop rounded tablespoons of dough onto parchment-lined baking sheets, spacing them about two inches apart.

Bake in the preheated oven for 10-12 minutes or until the edges are golden brown. Allow the cookies to cool on the sheets for a few minutes before transferring them to a wire rack.

For the filling, combine the marshmallow cream and powdered sugar in a medium bowl until well mixed.

Take one cooled cookie, spread the filling on the bottom, and top it with another cookie to create a sandwich.

Repeat this process with the remaining cookies.