• Whisk together the flour, salt, baking powder, baking soda, and cornstarch in a bowl and set aside.
  2 ¼ cups all-purpose flour, ¾ tsp salt, ½ teaspoon baking powder, ¼ tsp baking soda, 1 tbsp cornstarch
• In a separate large mixing bowl, cream the butter and sugar until fluffy, about 3 to 4 minutes.
  ½ cup unsalted butter room temperature, 1 cup granulated sugar
• Mix in the egg, vanilla extract, and sour cream, mixing until combined, scraping the bowl down as needed.
  1 large egg , ¼ cup sour cream , 1 ½ tsp vanilla extract
• Add the dry flour mixture to the wet butter and sugar mixture, and mix on low until just combined.
• Tip the dough out and shape into a disk. Wrap with plastic wrap and refrigerate for 1 hour.
• Preheat the oven to 375° F. Line twp rimmed cookie or baking sheets with parchment paper.
• Roll the dough out to ⅜ inch then use a 2 ½-3 inch circular cookie cutter to cut the cookies, re-rolling scraps for additional cookies.
• Transfer the cut out cookies to the prepared baking sheets and bake at 375°F for 8-9 minutes, or until the cookies are just turning color on the bottom edge. Set aside to cool completely.
Don't over bake! These cookies will be very light and the browning edges should just be visible.
• Make the frosting while the cookies are baking, but do not frost them until completely cooled. Add sprinkles to finish off.

For the Buttercream Frosting

• Using a medium mixing bowl, cream the butter and powdered sugar together. Add the vanilla extract and mix in.
  ½ cup unsalted butter , 2 cups powdered sugar, 1 tsp vanilla extract
• Add the milk, salt, and mix until fully blended and smooth. Add any food coloring if making tinted frosting.
  2 tbsp Milk, ½ teaspoon salt, Food coloring
• Place a tablespoon of frosting in the center of each cookie. Use a butter knife to spread it outward in a circular, swirling motion. Add sprinkles if using. Set on a wire rack and repeat with remaining cookies.
  Sprinkles