Ingredients
Equipment
Method
Make the dough
- Heat the oven to 350°F.
- Line two rimmed baking sheets with parchment paper.
- Beat the butter and 2/3 cup of the granulated sugar in a large bowl with an electric mixer on medium speed until light and fluffy, about 2 minutes.
- Beat in the egg yolk and vanilla until combined.
- Whisk the flour, cream of tartar, baking soda, and salt in a medium bowl.
- Mix the dry ingredients into the butter mixture on low speed just until no dry flour remains.
- Cover and refrigerate the dough for 30 minutes.
Shape and bake
- Stir the remaining 1/4 cup granulated sugar and the cinnamon in a shallow bowl.
- Roll tablespoon-size portions of dough into balls, coat them in the cinnamon sugar, and arrange them about 2 inches apart on the prepared sheets.
- Press a thumb into the center of each ball to make a deep well.
- Bake for 10–12 minutes, swapping the sheets’ positions and rotating them halfway through, until the edges are set and the bottoms are lightly golden.
- Cool the cookies on the sheets for 5 minutes.
- Gently press the wells again if they have puffed up.
- Transfer the cookies to a wire rack to cool completely.
Fill the cookies
- Whisk the powdered sugar, eggnog, and nutmeg in a bowl until smooth and thick.
- Spoon about 1 teaspoon into each cooled cookie’s well.
- Refrigerate the filled cookies for 30 minutes, until the filling is set.
- Store leftovers covered in the refrigerator.
Notes
For neat centers, press the wells again while the cookies are warm and fill only after they cool completely. Store covered in the refrigerator for up to 5 days; freezing is possible, preferably before filling. For a dairy-free variation, use dairy-free butter and plant-based eggnog.