In a medium bowl, whisk together flour and next 8 ingredients. Set aside.
Using an electric mixer, beat butter, shortening, and brown sugar at medium-high speed until creamy, about 3 minutes.
Add egg, molasses, and vanilla. Scrape down sides of bowl and beat until evenly combined.
Turn mixer to low speed and gradually add flour. Once combined, turn mixer off, cover dough with plastic wrap and refrigerate for at least 2 hours.
Preheat oven to 350 degrees and line 2 baking sheets with parchment paper.
Use a 1-inch spring-loaded ice cream scoop to scoop dough into mounds and place 3 inches apart on baking sheets.
Use your hands to roll each mound of dough into a ball. Roll balls in sugar and place back on baking sheet.
Bake 10 to 12 minutes. Edges should be firm and centers cracked. Let cool on pans several minutes before removing to wire racks to cool completely.
For Marshmallow Filling, beat marshmallow crème and shortening at medium-high speed with a mixer until smooth. Mix in maple syrup. With mixer turned to low speed, gradually add confectioners' sugar.
Place filling in a pastry bag fitted with a large star tip and pipe filling onto flat side of half the cookies. Pair together with remaining cookies to make sandwiches.
Alternately, you can spread the filling using an off-set spatula.
Notes
If you only have salted butter, use it and reduce the added salt slightly.