Gingerbread Cookie Dough:
In a large bowl, place the Betty Crocker Gingerbread cookie mix and the ingredients as called for on the back of the package. To that bowl, add the additional flour, molasses, and vanilla extract, stir and combine until a soft dough forms.
Shape the dough into a disc and wrap it tightly in plastic wrap. Refrigerate for 1 hour to firm up the dough and help it roll out cleanly.
After chilling, preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
Lightly flour your work surface and roll the chilled dough to ¼-inch thickness. Cut out cookies using your preferred cutter and transfer them to baking sheets, spacing them about an inch apart.
Bake the cookies for 12-14 minutes, until the edges look set but the centers are still soft. Cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Make the Marshmallow Filling:
In a medium size bowl using an electric mixer, beat the cream cheese until smooth. Add the marshmallow fluff, vanilla extract, salt and mix until combined. Gradually add powdered sugar, mixing until the filling is light and fluffy.
Assemble the Sandwich Cookies:
Once the cookies are completely cool, pipe or spread the marshmallow filling on the flat side of half the cookies. Top with the remaining cookies to form sandwiches. Finish with a dusting of powdered sugar if desired.