In a large mixing bowl, whisk together flour, baking soda, salt, cinnamon, pumpkin pie spice, and oats. Set aside.

Beat butter, brown sugar, and white granulated sugar until creamy and light.

Add vanilla extract, molasses, and eggs to the mixture. Mix until fully combined.

Gradually add dry ingredients to wet ingredients until just combined.

Chill the dough for 15 minutes and preheat oven to 350°F (175°C).

Scoop out dough to form about 36 cookies on baking sheets.

Bake cookies for 8-9 minutes, let cool on the sheet for 4 minutes.

Beat butter and marshmallow fluff until fluffy. Add powdered sugar, vanilla extract and salt, mix until airy.

Spread frosting on the flat side of one cookie, press another cookie on top.

Chill the assembled pies for at least 10 minutes in the freezer.

Store leftovers in an airtight container in the refrigerator.