1. Cream the butter and sugar together in a large bowl until light and fluffy.

2. Add the eggs one at a time, mixing well after each.

3. Stir in the sour cream and vanilla extract until smooth.

4. In another bowl, whisk together the flour, baking powder, cornstarch, and salt.

5. Gradually add the dry ingredients to the wet mixture until a soft dough forms.

6. Wrap the dough in plastic wrap and chill in the refrigerator for at least 1 hour.

7. Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper.

8. Roll the dough on a floured surface to ¼-inch thickness. Cut into shapes with cookie cutters.

9. Place cookies on the baking sheet and bake for 8–10 minutes, or until edges are barely golden.

10. Let cookies cool completely on a wire rack.

11. Make the icing by whisking together powdered sugar, milk, and vanilla extract. Adjust consistency if needed.

12. Ice the cooled cookies and decorate with sprinkles while the icing is still wet.

13. Let icing set before stacking or storing.