• Make the cookie dough. In a medium-sized bowl, cream the softened butter, sour cream, vanilla, and powdered sugar. Use an electric mixer or whisk. Scraping the bowl with a spatula and mix until slightly creamy. Then add the flour, cornstarch, and salt. Mix until incorporated. Do not overmix.

  Shape the dough into a ball and flatten it into a disk. Wrapped and refrigerate for at least an hour. If making ahead refrigerate overnight.
• Roll, cut, and refrigerate the cookies. Line the baking sheet with parchment paper. Roll out the cookie dough on parchment paper or lightly floured work surface to about 1/4″ thick. Cut with the star (size 3″ or 4″) or any cookie cutter that you have. Place the cookie dough cutouts on the baking sheet lined with parchment paper. Reroll the dough scraps or trimmings as needed.

  Refrigerate for at least 30 minutes or place in the freezer for 10 to 15 minutes before baking. 
• Bake the cookies. Position the oven racks on top and bottom. Preheat the oven to 350°F. Bake in preheated oven for about 12 to 15 minutes or until they start slightly browning around the edges of the cookies. Halfway through the baking time, rotate the baking sheet and switch from top to bottom to ensure the even cooking of the cookies.
• Cool the cookies. Transfer the cookies to a cooling rack. While your waiting for the cookies to cool completely, make the icing.
• Make the Icing. Mix all the icing ingredients in a bowl until smooth. Either use a whisk and mix by hand or an electric mixer.

Decorate the cookies

• Transfer the icing to a piping bag and decorate it the way you like it. You can use the piping bag with or without a tip.
• Add the sprinkles to a small platter, then dip the cookies with the icing. Fix the edges with a toothpick or cake tester. Let it dry before adding and piping numbers or letters on top. 
• Allow the icing to dry completely or until hard before stacking them in a container or cookie box.