First Step: Prepare the Cookie Dough

1. Mix the Dry Ingredients: In a medium-sized bowl, whisk together the flour, baking soda, cream of tartar, salt, and cinnamon. Set aside.
2. Cream the Butter and Sugar: In a large mixing bowl, cream together the softened butter and granulated sugar using an electric mixer on medium speed until light and fluffy (about 3-4 minutes).
3. Add the Eggs and Vanilla: Beat in the eggs, one at a time, making sure each is fully incorporated before adding the next. Add the vanilla extract and continue mixing until smooth.
4. Combine Dry and Wet Ingredients: Gradually add the dry ingredients to the butter mixture, mixing just until combined. Be careful not to overmix, as this can result in tough cookies.
5. Chill the Dough: Cover the dough with plastic wrap and refrigerate it for at least 30 minutes. Chilling the dough helps prevent the cookies from spreading too much while baking.

Second Step: Prepare the Cinnamon-Sugar Coating

1. Mix the Coating: In a small bowl, combine the granulated sugar and ground cinnamon. Stir until well mixed and set aside. This coating will give your cookies the perfect snickerdoodle finish.

Third Step: Shape and Coat the Cookies

1. Preheat the Oven: While the dough is chilling, preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat for easy cleanup.
2. Shape the Dough: Once the dough is chilled, scoop out tablespoon-sized portions and roll them into balls. Roll each dough ball in the cinnamon-sugar mixture, making sure it’s fully coated.
3. Create the Thumbprint: Place the dough balls onto the prepared baking sheet, spacing them about 2 inches apart. Gently press your thumb into the center of each cookie to create a small indentation. This will hold the eggnog glaze later.

Fourth Step: Bake the Cookies

1. Bake the Cookies: Bake the cookies at 350°F (175°C) for about 8-10 minutes or until the edges are golden and the centers are set. Be careful not to overbake them, as they should remain soft and chewy.
2. Cool the Cookies: Once baked, remove the cookies from the oven and allow them to cool on the baking sheet for about 5 minutes. Then, transfer the cookies to a wire rack to cool completely.

Fifth Step: Prepare the Eggnog Glaze

1. Mix the Glaze: In a small bowl, whisk together the eggnog, powdered sugar, cinnamon, nutmeg, and vanilla extract. The glaze should be smooth and pourable. If it’s too thick, you can add a little more eggnog to reach your desired consistency.
2. Fill the Thumbprints: Once the cookies are completely cool, spoon a small amount of the eggnog glaze into the thumbprint indentation of each cookie. Be careful not to overfill, as the glaze may spill over.

Sixth Step: Garnish and Serve

1. Garnish (Optional): For an extra festive touch, sprinkle a pinch of cinnamon or nutmeg over the glaze before serving. This will enhance the flavor and add to the holiday aesthetic.
2. Serve: Once the glaze is set, your Eggnog Snickerdoodle Thumbprint Cookies are ready to serve! These cookies are perfect for holiday parties, cookie exchanges, or as a sweet treat to enjoy with a warm cup of tea or coffee.