Preheat your oven to 350°F. Line 2 baking sheets with parchment paper or a silicone baking mats. Set aside.
In a medium-sized bowl, combine the flour, baking soda, spices, and salt. Mix well. Set the bowl aside.
Beat the softened butter, molasses and granulated sugar together. Add in the egg and vanilla extract and mix again until mixture is well combined.
Add the flour mixture to the bowl and begin mixing. While mixing on a low speed, slowly pour in the milk. Mix just until everything is combined and the dough is smooth. Do not overmix.
Use a medium-sized cookie scoop to scoop out the dough. Place dough about 3 inches apart on the prepared baking sheets. Bake for 11-13 minutes. The cookies should have puffed up and but should still be slightly soft to the touch. Allow them to cool on the baking sheet for about 5 minutes, then carefully transfer them to a cooling rack to finish cooling completely.
Making the Filling
To make the filling, cream together the softened butter and softened cream cheese until smooth. Next, add in the vanilla extract, marshmallow fluff, and salt. Once combined, add in the powdered sugar. Start by adding 1 ½ cups. Add an additional ¼ cup of powdered sugar at a time if you need/want your filling to be firmer until you reach your desired consistency. Once done, turn your mixer up to a medium speed and let it whip for about a minute to get it light and fluffy.
Using a piping bag, plastic storage bag, or spoon, place equal amounts of filling on the underside of half of the cookies. Top each on with an unfilled cookie, creating a sandwich. Enjoy!