Ginger Molasses Cookies:
In a medium bowl, whisk together the flour, spices, baking soda, and salt.
In a large bowl, beat together the butter and brown sugar until fluffy.
Beat in the egg, vanilla extract, and molasses.
With the mixer on low speed, gradually add the flour mixture into the butter mixture until combined.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes or overnight.
When ready to bake, preheat the oven to 350°F (180°C) and line cookie sheets with parchment paper or baking mats.
Pour the white sugar onto a small plate.
Form the dough into balls with about 1 – 1.5 tablespoons of dough each (about 1 inch in diameter) and roll them in the sugar. Place cookie dough balls about 2 inches apart on the prepared cookie sheets.
Bake the cookies one sheet at a time on the middle rack of the oven for about 8-10 minutes, or until the tops look set.
Marshmallow Frosting:
In a large bowl, beat the butter until fluffy.
Beat in the marshmallow cream and salt.
Mix in the powdered sugar about 1/2 cup at a time, adding in a little milk or cream until the desired sweetness and consistency are reached.
Assembling the Cookies:
Once the cookies are fully cooled, frost the bottom of one cookie with a bit of frosting and then gently sandwich a second cookie on top.