Beat the softened butter until it is light and fluffy. 

In a separate bowl, mix together the cornstarch, powdered sugar, and flour.

Slowly add the dry ingredients to the butter and mix until moistened. It may take a couple of minutes for it to start to come together. You can check if it is moist enough by pinching some dough with your fingers. If it holds together, it is ready. If it is still too dry, add a teaspoon of milk.

Use a small cookie scoop (about 1 tablespoon) to measure the cookies, then roll dough between your hands to make a ball. Place them on a prepared baking sheet lined with parchment paper about 2 inches apart and gently flatten the dough slightly with the bottom of a glass. Make a small well in the center of the cookie using your thumb or finger.

Bake the cookies at 350 degrees for 10 minutes making sure to watch them carefully to avoid over baking. Remove them from the oven and transfer them to a cooling rack after 1 minute.

Beat the softened cream cheese in a small bowl or food processor, until smooth. Add the vanilla and the powdered sugar a little at a time. Mix until incorporated. If desired, add food coloring to the frosting now.

Use a piping bag or a Ziplock bag with the corner cut off, to add the frosting to the tops of the cookies. Top with sprinkles.