How to Make It
Preheat oven to 350°F. Beat butter with a heavy-duty stand mixer fitted with paddle attachment on medium speed until creamy; gradually add granulated sugar and brown sugar, and beat until light and fluffy, about 3 minutes. Add egg yolks, 1 at a time, beating just until incorporated after each addition. Beat in eggnog and vanilla until incorporated.
Whisk together flour, baking powder, salt, nutmeg, and cinnamon in a small bowl. With mixer running on low speed, gradually add flour mixture to butter mixture, beating just until incorporated. Using a 1 1/2-tablespoon cookie scoop, drop dough 2 inches apart onto parchment paper-lined baking sheets.
Bake in preheated oven until cookies are very lightly browned around edges, 11 to 12 minutes. Cool cookies on baking sheets on a wire rack 5 minutes. Remove cookies to wire racks to cool completely, about 30 minutes.
Spread top of each cookie with about 2 teaspoons Eggnog-Bourbon Buttercream, and, if desired, sprinkle with nutmeg. Store cookies in an airtight container for up to 3 days.