PREHEAT oven to 325F. Line 2 baking sheets with parchment.
WHISK flour, cornstarch and salt in a bowl.
BEAT butter and sugar in a large bowl using an electric mixer on medium-high until very fluffy and aerated, 4 to 5 min, scraping sides and bottom of bowl. Add milk and vanilla and beat for 1 min.
SIFT in half of the flour mixture, then fold in with a spatula until just combined. Repeat. Spoon half of the batter into a piping bag fitted with a large, open star tip.