Preheat the oven to 350°F (175°C).

In a bowl, combine flour, baking soda, cream of tartar, salt, cinnamon, and nutmeg.

In a separate bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs and vanilla extract to the butter mixture, mixing well.

Gradually add the dry mixture to the wet ingredients, mixing until just combined.

Incorporate the eggnog into the dough.

Roll the dough into balls and place them on a baking sheet.

Make an indentation in the center of each ball using your thumb.

Bake for 12 minutes or until the edges are lightly golden.

In a bowl, mix powdered sugar, softened butter, eggnog, and vanilla extract to make the icing.

Once the cookies have cooled, fill the thumbprint with the icing and garnish with ground nutmeg if desired.