In a mixing bowl, whisk together the all-purpose flour, baking soda, salt, ground cinnamon, pumpkin pie spice, and old fashioned oats until thoroughly combined.

In a large mixing bowl, beat the unsalted butter, brown sugar, and white granulated sugar together until light and fluffy, about 2-3 minutes.

Incorporate the large eggs, pure vanilla extract, and molasses into your butter mixture. Mix until smooth, about 1-2 minutes.

Gradually add your dry ingredient mixture to the wet ingredients and gently fold together until just combined.

Cover the dough with plastic wrap and refrigerate for at least 45 minutes.

Once chilled, scoop rounded tablespoons of dough onto lined baking sheets, spaced about 2 inches apart. Bake at 350°F for 9-11 minutes.

Prepare the marshmallow cream filling by beating together the marshmallow fluff and unsalted butter until smooth, then gradually mix in the powdered sugar.

To assemble, pipe a generous amount of marshmallow filling onto one cooled cookie, then top with another cookie.