Preheat the oven to 400°F (204°C) and line two baking sheets with parchment paper. Using parchment helps the cookies bake evenly and prevents excess browning on the bottoms.
In the bowl of a stand mixer fitted with the paddle attachment, cream the softened butter and granulated sugar together on medium speed until light and fluffy, about 2 minutes. The mixture should look pale and aerated, which helps create a tender cookie structure.
1 cup (227 g) unsalted butter, 1½ cups (330 g) granulated sugar
Next, add the egg, egg yolk, vanilla extract, rum extract, and eggnog. Mix until smooth and fully combined, scraping down the sides of the bowl as needed to ensure everything is evenly incorporated.
1 large egg, 1 egg yolk, 1 tsp (5 ml) vanilla extract, ½ tsp (2.5 ml) rum extract, 3 tbsps (45 ml) eggnog
Meanwhile, in a separate medium bowl, whisk together the all-purpose flour, cream of tartar, baking soda, fine sea salt, ground nutmeg, and ground cinnamon. Whisking thoroughly distributes the leavening and spices evenly throughout the dough.
2¾ cups (374 g) all-purpose flour, 2 tsps (6 g) cream of tartar, 1 tsp (5 g) baking soda, ½ tsp (3 g) fine sea salt, ½ tsp (1 g) ground nutmeg, ¼ tsp (0.5 g) ground cinnamon
With the mixer set to low speed, gradually add the dry ingredients to the wet ingredients, mixing just until combined. Stop as soon as no dry streaks remain. The dough should be soft and slightly sticky but cohesive. Overmixing at this stage can lead to dense cookies.
To prepare the coating, combine the granulated sugar, ground cinnamon, and ground nutmeg in a small bowl and mix well.
¼ cup (55 g) granulated sugar, 1½ tsps (4 g) ground cinnamon, ½ tsp (1 g) ground nutmeg
Using a cookie scoop or measuring spoon, portion the dough into 2-tablespoon-sized balls (about 30 ml). Roll each portion gently between your hands to form a smooth ball, then roll thoroughly in the sugar-spice mixture until fully coated.
Arrange the dough balls on the prepared baking sheets, spacing them about 2 inches (5 cm) apart to allow for spreading.
Bake for 8 to 10 minutes, rotating the pans halfway through if your oven has hot spots. The cookies are ready when the edges are set and the tops are crackled, while the centers still look slightly soft. Avoid overbaking, as the cookies will continue to set as they cool.
Once baked, allow the cookies to rest on the baking sheets for about 5 minutes before transferring them to a wire rack to cool completely. This short rest helps the cookies firm up without drying out.
If using the optional glaze, whisk the confectioners’ sugar and eggnog together in a small bowl until smooth and pourable. Once the cookies are fully cooled, drizzle the glaze lightly over the tops and let it set before serving or storing.
½ cup (60 g) confectioners' sugar, 1 tbsp (15 ml) eggnog