Sift together the flour, cornstarch, baking powder, baking soda, and salt. Set aside.

Using a hand mixer on medium speed, beat the confectioners’ sugar and butter until light in color and crumbly. Mix in the egg, sour cream, and vanilla until completely incorporated.

Add the flour mixture to the butter mixture, and mix on low speed until just barely combined. Stir a few times with a spatula to finish mixing the dough. Cover and refrigerate for one hour. 

Roll the chilled dough out to 1/4-inch thickness on a clean floured surface. Use a 3-inch round cookie cutter to cut out 20 cookies. Re-roll as needed until all dough has been cut. Place the cookies on the prepared pans.

Bake one pan at a time for 10 to 12 minutes. The cookies will be light in color.

Place the pans on a wire rack, and allow to cool for 5 minutes. Then transfer the cookies from the pan directly onto a rack to cool completely.

In the bowl of a stand mixer with the paddle attachment, beat the butter on a medium speed for 3 to 5 minutes. The butter will be light in color. Add the vanilla, salt, and half of the cream. Gradually add the confectioners’ sugar and food coloring (if using), and then add the remaining cream. 

Continue beating the frosting for 5 more minutes. Then, using a spatula, press the frosting around the sides of the bowl to release any air bubbles for the best consistency.

Spread or pipe the frosting onto the cooled cookies, leaving a small rim of the cookie edge visible. Add sprinkles.