In a large bowl, cream butter and powdered sugar until VERY light and fluffy, about 5 minutes on medium speed. Stir in vanilla.
On low speed, gradually add flour, salt and cornstarch until blended.
With hands lightly dusted with additional cornstarch, pinch off pieces of dough and roll into 1-inch balls. Place on parchment paper-lined baking sheets. Press lightly with a fork dipped in cornstarch to make impression. Sprinkle with nonpareils or jimmies.
Bake at 300 degrees F for 20-22 minutes or until bottoms are golden and tops are set. Cool for 5 minutes on baking sheet before removing to wire racks to cool completely.