Sift together flour and next 3 ingredients in a medium bowl; stir in graham cracker crumbs.
Beat butter and next 2 ingredients at medium speed with a heavy-duty electric stand mixer until fluffy.
Add egg and vanilla, beating until blended.
Add flour mixture to butter mixture alternately with sour cream, beginning and ending with flour mixture. Beat at low speed until blended after each addition, stopping to scrape bowl as needed.
Drop batter by rounded tablespoonfuls 2 inches apart onto 2 parchment paper-lined baking sheets.
Bake, in batches, at 350 for 13 to 15 minutes or until set and bottoms are golden brown.
Remove cookies (on parchment paper) to wire racks, and cool completely (about 30 minutes).
Turn 12 cookies over, bottom sides up.
Spread each with 1 heaping tablespoonful Marshmallow Filling. Top with remaining 12 cookies, bottom sides down, and press gently to spread filling to edges. Freeze on a parchment paper-lined baking sheet 30 minutes or until filling is set.
Pour water to depth of 1 inch in a medium saucepan over medium heat; bring to a boil. Reduce heat, and simmer; place chocolate and shortening in a medium-size heatproof bowl over simmering water. Cook, stirring occasionally, 5 to 6 minutes or until melted.
Remove from heat, and let cool 10 minutes.
Meanwhile, remove cookies from freezer, and let stand 10 minutes.
Dip each cookie sandwich completely into melted chocolate mixture. (Use a fork to easily remove sandwiches from chocolate.)
Place on parchment paper-lined baking sheet.
Sprinkle with desired toppings, and freeze 10 minutes or until chocolate is set.
Note: Cookie sandwiches may be covered with plastic wrap and stored in refrigerator up to 24 hours.
