Preheat your oven to 400 degrees F (200 degrees C).

Line a baking sheet with parchment paper.

In a large mixing bowl, whisk together the flour, oats, pumpkin pie spice, baking powder, baking soda, salt, and nutmeg until evenly combined.

In a stand mixer, beat the room-temperature butter and brown sugar together on low speed until smooth and creamy, about 2-3 minutes.

Add the eggs, molasses, and vanilla to the butter-sugar mixture. Beat until fully incorporated.

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

Using a cookie scoop or two spoons, drop scoops of dough onto the prepared baking sheet, leaving space between each cookie.

Lightly flatten the tops with your fingers.

Bake in the preheated oven for about 7-8 minutes, or until the edges are set but the centers still look a bit doughy.

Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

In a mixing bowl, beat the softened butter and marshmallow fluff together until smooth.

Gradually add the confectioners' sugar until stiff and creamy.

Once cookies are completely cool, spread a generous dollop of marshmallow filling onto the flat side of one cookie and top with a second cookie, flat side down.

Your delicious Pumpkin Spice Oatmeal Cream Pies are ready to enjoy! Store extras in an airtight container.