Rich and delicious eggnog buttercream frosting snuggled inside of Snickerdoodle Sandwich Cookies makes for a perfect holiday treat!
Preheat your oven to 400°F. Cover two cookie sheets with parchment paper or a silicone baking mat.
In a stand mixer, using the paddle attachment, cream butter, shortening, and sugar. Beat in egg and vanilla.
Scrape down the sides and bottom of the bowl. On low, mix in flour, cream of tartar, baking soda, and salt until just combined.
In a small bowl, mix 2 tablespoons of sugar with the cinnamon. Scoop 36 1/2-ounce mounds of dough into balls (just over 1-inch wide) and roll in sugar mixture. Place on a baking sheet 1 1/2-inches apart. These are small cookies, so you should be able to fit 18-20 on a standard-size cookie tray.
Bake for 6 to 8 minutes. They are done when they are just golden and look set. You don't want them to crisp up too much since we are turning them into sandwiches.
Allow cookies to cool completely.
In a small bowl, whisk together powdered sugar, nutmeg, and salt. Set aside. Cream butter and add the powdered sugar mixture and 1 tablespoon of the eggnog, beating until combined. If your frosting is too dry, mix in a little more eggnog, 1 teaspoon at a time, until you reach the desired consistency.
Pair cookies by size so that you have 18 sandwiches with matching halves.