In large bowl, mix powdered sugar & butter together.
Mix in egg & vanilla until smooth.
Stir in flour, baking soda, cream of tartar & cinnamon until well blended. Cover & refrigerate for 1 hour or until firm.
Preheat oven to 375°F (if using dark or nonstick cookie sheet, heat oven to 350°F).
Divide dough in half. On lightly floured surface, roll half of dough at a time to ¼ inch thick. (I like to place a piece of waxed paper over the top when I'm rolling out the dough so it doesn't stick to the rolling pin).
Cut with 2-inch star-shaped cookie cutter.
Place on cookie sheets, 1 inch apart. Sprinkle with a mixture of cinnamon & sugar.
Bake 7 to 8 minutes or until light golden.
Recipe by THE SEMISWEET SISTERS at https://www.thesemisweetsisters.com/2015/12/05/christmas-cinnamon-star-shortbread-cookies/