In a mixing bowl, use an electric beater to mix the butter and sugar until light and fluffy. Beat in the egg yolks and rum flavoring.
In a second bowl, combine the flour, cream of tartar, baking soda, salt, cinnamon, and nutmeg. Mix the dry ingredients into the wet ingredients. Cover and chill 30 minutes.
Heat oven to 400F and line two baking sheets with a silpat or parchment paper. In a small bowl, combine the remaining sugar and nutmeg.
For teaspoon-sized portions of dough into balls; roll in the spiced sugar and place on the prepared baking sheets 2 inches apart. Bake 10-12 minutes, or until the cookies are cracked and the top looks dry. Cool on a wire rack.
Repeat with remaining cookie dough.