In a stand mixer, beat together unsalted butter, granulated sugar, and brown sugar until light and fluffy, about 3-4 minutes.

Add dark rum and vanilla extract, mixing for an additional 30 seconds.

Incorporate the whole egg and egg yolks, beating until fluffy, about 2 minutes.

In a separate bowl, whisk together the flour, cream of tartar, baking powder, salt, cinnamon, and nutmeg.

Gradually add the dry mixture into the wet mix, mixing on low speed.

Pour in eggnog until the dough is slightly sticky.

Using a scoop, portion the dough onto a lined baking sheet and refrigerate for at least 1 hour.

Preheat the oven to 350°F (175°C), mix together the sugar, cinnamon, and nutmeg for coating.

Roll chilled dough balls in the sugar mixture before placing on a baking sheet.

Bake for 15-17 minutes until puffy and barely golden around the edges.

Let cool on the baking sheet for a few minutes before transferring to a wire rack.