Preheat oven to 350. Line two baking sheets with parchment paper. In a large bowl, beat butter and sugar until light and fluffy. Add vanilla extract and mix until blended. Add flour and cornstarch, and mix thoroughly.
Spoon mixture into a pastry bag fitted with a large star tip (I used the Wilton #806 round tip). Pipe cookies onto baking sheet and press sprinkles into tops, and flatten slightly. Bake for 15 minutes, or until golden brown.