Preheat the oven to 350°F. Line a large baking sheet with parchment paper and set aside.: The moment you preheat the oven you begin the setup for the right texture, because a properly heated oven gives a consistent spread and edge browning. You should feel a warm wave when you open the oven door, and the oven should register a steady 350°F on the dial. If you skip preheating the cookies may bake unevenly, leading to centers that remain underdone while edges overbrown. A common mistake is not allowing the oven to fully reach temperature, so wait until the indicator light shows it is ready.
In a large mixing bowl or the bowl of a stand mixer, beat together butter, brown sugar and granulated sugar until light and creamy, scraping down the sides and bottom of bowl as necessary.: Lining the sheet prevents sticking and makes cleanup simple, and parchment gives a slightly insulated surface that promotes even browning. When you press the parchment it should lie flat and not ripple; if it does, smooth it out so the cookies bake evenly. Avoid greasing the parchment, as that can cause excess spreading. People often forget to set the prepared sheet aside, which leads to pauses while the dough waits and can cool the butter too much.
Add in the egg yolk, vanilla extract and eggnog and beat until smooth and combined, scraping down the sides and bottom of bowl as necessary.: As you beat the butter and sugars you will see the mixture turn paler and fluff up, which traps air that helps the cookies rise and create a tender bite. The sound changes from a slosh to a gentle whir as the mixture becomes smooth. Scrape down the sides and bottom of the bowl so all ingredients incorporate evenly; otherwise you risk pockets of unmixed butter or sugar. A common error is under beating, which leads to dense cookies, or over beating, which can warm the butter too much.
In a separate bowl, whisk together all purpose flour, ground nutmeg, ground cinnamon, baking soda, baking powder and salt.: When you add the egg yolk , vanilla , and eggnog , the mixture should appear glossy and homogeneous. You may notice a faint fragrance of dairy and spice as the eggnog blends in. If the mixture seems curdled, it usually smooths out with a bit more beating; however do not overwork the dough. One slip is adding cold eggnog or egg yolk, which can cause the butter to seize, so ensure these are room temperature.
Add the flour mixture in two additions, mixing just until combined.: In the separate bowl the all purpose flour and spices should be evenly mixed, so each bite has a balanced flavor. You will see a uniform tan color when the ground nutmeg and ground cinnamon are well distributed. Sifting or whisking also helps break up any lumps in the leaveners. A common misstep is not mixing the dry ingredients thoroughly, which can leave pockets of baking powder or soda, causing uneven rise and flavor.
Scoop out cookie dough, roll into 1 inch balls (dough will be a bit sticky) and place them on the prepared baking sheet.: Adding the dry mix in two parts helps avoid over mixing, which preserves a tender texture. After each addition stir just until you no longer see streaks of flour, the dough should look cohesive but still slightly soft. Over mixing will develop gluten and make the cookies tough, so stop as soon as the dough comes together. Many home bakers keep stirring until perfect uniformity, but that extra mixing can harm the final bite.
Bake the cookies for 11 to 13 minutes or until just beginning to turn golden brown on the edges and the centers no longer appear wet.: The dough will be a bit sticky, and when you roll it into 1 inch balls you should feel a slight tackiness that indicates moisture and tenderness. Place the balls with space between them so they can spread slightly, and if the dough is too sticky chill briefly to make rolling easier. If you press the balls too flat they will bake thin and crispy rather than soft and tender, which some people prefer but is not the intended texture here.
Remove from the oven, let sit on the baking sheet for a few minutes and then transfer to a wire cooling rack to cool.: While baking you will hear a quiet oven hum, and the edges will shift from pale to a warm golden brown, which signals they are ready to come out. The centers should look set rather than glossy; they continue to firm as they cool. Leaving them in too long will make them dry, and underbaking leaves a raw center, so use visual cues more than the timer if your oven runs hot or cool.
In a medium bowl, add cream cheese, eggnog and powdered sugar. Beat until nice and smooth.: Letting the cookies rest on the hot sheet allows carryover cooking to finish the centers while the edges stabilize. You will feel a gentle heat when you touch the rack after a minute or two. If you pull them off the sheet immediately they may fall apart, and if you leave them on the sheet too long they can overcook from the residual heat. Use a thin spatula to lift them to the rack carefully to avoid breaking.
Frost the cooled cookies with a knife or piping bag. Enjoy!: The cream cheese , extra eggnog , and powdered sugar should form a glossy, smooth frosting that spreads easily. The texture should be thick but pipeable; if it looks grainy it needs more beating or sifting of powdered sugar. Overbeating can incorporate air and make the frosting too loose, while underbeating leaves lumps of cream cheese.
Frost the cooled cookies with a knife or piping bag.: The final step is a tactile pleasure, smoothing the frosting until it looks inviting and slightly glossy. Spread or pipe the frosting when cookies are completely cool so it holds shape and does not melt. A frequent error is frosting warm cookies, which leads to a runny finish and loses the defined look.
Enjoy!: Take a moment to savor the aroma and texture, and share them while warm or store according to preference. If cookies are stored improperly they can become stale; keeping them in an airtight container preserves freshness. I love pairing a cookie with a warm cup of tea while they are still slightly soft in the center.