Making Eggnog Snickerdoodle Thumbprint Cookies is a straightforward process. Follow these steps to create your delicious cookies:

1. Preheat Oven: Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking soda, cream of tartar, salt, and ground cinnamon. Set aside.
3. Cream Butter and Sugar: In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
4. Add Eggs and Extracts: Beat in the eggs, vanilla extract, and rum extract (if using) until well combined.
5. Combine Mixtures: Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined.
6. Prepare Coating: In a small bowl, combine ¼ cup of granulated sugar and 1 tablespoon of ground cinnamon for the coating.
7. Shape Cookies: Scoop tablespoon-sized portions of dough and roll them into balls. Roll each ball in the cinnamon-sugar mixture to coat.
8. Make Thumbprints: Place the coated dough balls on the prepared baking sheets. Using your thumb or the back of a teaspoon, create an indentation in the center of each ball.
9. Bake: Bake in the preheated oven for 12-15 minutes or until the edges are lightly golden. The centers may look slightly underbaked, but they will firm up as they cool.
10. Prepare Filling: While the cookies are baking, combine the eggnog, cornstarch, vanilla extract, and nutmeg in a saucepan over medium heat. Stir constantly until the mixture thickens, about 5-7 minutes. Remove from heat and let cool slightly.
11. Fill Cookies: Once the cookies are slightly cooled, use a small spoon or piping bag to fill the thumbprint indentations with the eggnog filling.
12. Cool Completely: Allow the cookies to cool completely on the baking sheets before transferring them to a wire rack.

With these simple steps, you’ll have a batch of delightful Eggnog Snickerdoodle Thumbprint Cookies ready to enjoy!