Preheat oven to 350 degrees F.

In a stand mixer fitted with paddle attachment, cream butter and sugars until light and fluffy, scraping down once.

Add egg and vanilla and mix until creamy and combined, scraping down once.

Add flour, baking soda, cream of tartar, and salt, mixing until a thick dough forms.

Scoop dough into 1 tablespoon sized balls and roll in cinnamon sugar. Place on parchment lined cookie sheet, pressing down the centers with a ? teaspoon or with your finger/thumb.

Bake in preheated oven for 7 minutes. Press down the centers again if they fill in during baking.

Make the filling by combining white chocolate, eggnog, and rum or rum extract in a bowl. Microwave at 15-second intervals, stirring in between, until chocolate is melted. Add grated nutmeg if desired.

Fill each cookie with about ? teaspoon of filling and top with freshly grated nutmeg. Allow filling to set at room temperature or refrigerate for quicker setting.