Combine flour, cornstarch, and salt in a bowl, stirring well with a whisk.
Using an electric mixer or by hand with a whisk beat together the powdered sugar, butter, cream cheese, and vanilla until light and fluffy (about 4 minutes). Then fold it into the flour mixture.
Stir until just combined (dough will be dry and crumbly). Shape dough into 1-inch balls. Place balls 2 inches apart on a parchment paper lined baking sheet. Bake at 375° for 10 minutes or until bottoms of cookies are lightly browned. Cool for 5 minutes on pan. Remove from pan; cool completely on a dish.
For the glaze: combine the powdered sugar and lemon juice, stirring with a whisk until smooth. Dip tops of cookies in glaze; place on a dish. Distribute rainbow sprinkles over top. Let cookies stand for 30 minutes or until glaze is set.