1. Whisk the flour, cornstarch, baking powder, and salt in a medium bowl.

2. In a large bowl, beat the softened butter and granulated sugar until light and creamy.

3. Beat in the egg, sour cream, and vanilla extract until smooth.

4. Add the dry ingredients to the wet ingredients and mix on low just until a soft dough forms.

5. Cover and chill the dough for 45 minutes.

6. Preheat the oven to 350°F and line baking sheets with parchment paper.

7. Scoop the dough into rounded portions and place them 2 inches apart on the baking sheets.

8. Bake for 9 to 11 minutes, until the edges are set and the tops look puffed but still pale.

9. Cool the cookies on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

10. Beat the frosting butter until creamy, then add powdered sugar, vanilla, and heavy cream or milk. Beat until fluffy.

11. Add food coloring if using, then spread frosting over each cooled cookie.

12. Add sprinkles right away and let the frosting set slightly before serving.