In a mixing bowl, mix the flour, baking soda, salt, cinnamon, and pumpkin pie spice, and oats. Mix until combined. Set aside.
Using a mixer, beat the butter, brown sugar, and sugar together on high speed for 2 minutes. Add in vanilla, molasses, and eggs. Mix until combined on medium speed.
Add in the dry ingredients and mix on low speed until just combined.
Chill the dough in the fridge for 15 minutes. Preheat oven to 350°F. Line 2 cookie sheets with parchment paper.
Scoop the dough using a cookie scoop. It will make 3 dozen cookies. Place 12 per cookie sheet and place the rest of the scooped dough balls in the fridge.
Bake for 8-9 minutes. Let the cookies sit on the cookie sheet for 4 minutes before transferring to a cooling rack. Let cookies cool completely before frosting.
Using a mixer, beat the butter and marshmallow cream on high speed for 3 minutes.
Add in the powdered sugar, vanilla, and salt on low speed until just combined. Beat on high speed for 4 minutes.
Pipe the frosting into 1 layer on the back of a cookie. Top with another cookie. Chill in the freezer for at least 10 minutes before serving. If you are serving later, store in the fridge.
