Preheat your oven to 350°F (180°C) and line your baking sheets with parchment paper.

Whisk together flour, baking powder, salt, nutmeg, and cinnamon in a bowl. Set aside.

Beat butter, granulated sugar, and brown sugar in a stand mixer until light and fluffy.

Add yolks one at a time, mixing after each. Add vanilla, rum extract, and eggnog, then mix well.

Gradually add dry ingredients to wet mixture at low speed until just combined.

Scoop rounded portions of dough onto baking sheets, spacing 2 inches apart.

Bake for 11-13 minutes, or until edges are lightly golden. Cool on sheets before transferring to a wire rack.

Whip butter for frosting until fluffy, then gradually mix in powdered sugar and enough eggnog to achieve smooth consistency.

Frost cooled cookies and sprinkle with additional nutmeg if desired.