• Preheat oven to 350F (175C) and line baking sheet with parchment paper. Set aside.
• In a large mixing bowl (or in the bowl of a stand mixer using the paddle attachment), use an electric mixer to beat butter and sugars until well creamed (about 2-3 minutes on high speed).
  1 cup unsalted butter, 1 cup granulated sugar, ⅓ cup light brown sugar
• Add egg and vanilla extract and beat until well combined.
  1 large egg, 1 teaspoon vanilla extract
• Stir in eggnog until well incorporated into the dough.
  ¼ cup eggnog
• In a separate, medium-sized mixing bowl, whisk together flour, baking powder, cinnamon, nutmeg, baking soda, and salt.
  3 ⅓ cups cake flour, 1 teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon ground cinnamon, ¼ teaspoon nutmeg, ¾ teaspoon table salt
• Gradually add dry ingredients to the wet (I do this in 3-4 parts), stirring well between each addition. Use a spatula to scrape the sides and bottom of the bowl to ensure all ingredients are well incorporated.
• Scoop dough by level 1 ½ Tablespoon-sized scoop and drop onto prepared baking sheet.
• Transfer to center rack of 350F (175C) preheated oven and bake for 10 minutes, or until edges are just begin turning a very light brown.
• Within a minute of the cookies coming out of the oven, use the bottom of a measuring cup (or similar) to very gently flatten the surface of each cookie. Allow to cool completely on baking sheet before covering frosting.

Frosting

• In a medium-sized mixing bowl, whisk together sugar and eggnog until smooth. Mixture will be thin like a glaze.
  3 cups powdered sugar, 6 Tablespoons eggnog
• Spread 1 heaping teaspoon of frosting evenly over the surface of each cookie (I use an offset spatula to spread the frosting, but a butter knife would work). Allow frosting to harden before enjoying.