01 -
Set oven to 350 degrees Fahrenheit. Line a baking sheet with parchment paper.
02 -
In a stand mixer with paddle attachment, cream the unsalted butter, light brown sugar, and white granulated sugar on medium speed until light and fluffy. Scrape down the bowl once during mixing.
03 -
Add the egg and vanilla extract to the creamed mixture. Mix until thoroughly combined, scraping down the bowl once.
04 -
Add kosher salt, baking soda, and cream of tartar. Mix until just blended and dough forms.
05 -
Scoop dough into 1-tablespoon portions and roll each into a ball. Combine 1/3 cup sugar with 2 tablespoons cinnamon in a shallow bowl. Roll dough balls in cinnamon sugar. Arrange on the prepared cookie sheet.
06 -
Press the center of each dough ball with your thumb or using a 1/2 teaspoon measure to create a deep indentation. If edges crack, gently press them back together.
07 -
Bake on the center rack for 10 to 12 minutes or until edges are lightly golden. Remove from oven and let cool completely on the baking sheet.
08 -
In a microwave-safe bowl or over a double boiler, melt white chocolate with eggnog. Stir until smooth. Add 1/4 teaspoon freshly grated nutmeg and mix until incorporated.
09 -
Spoon approximately 1/2 teaspoon of eggnog filling into the center of each cooled cookie. Sprinkle with extra grated nutmeg for garnish.
10 -
Allow cookies to stand at room temperature for several hours until filling is set. For quicker results, place in the refrigerator.