1. Beat butter + sugar until fluffy.
2. Mix in egg + vanilla.
3. Add flour, cornstarch, salt → soft dough.
4. Pipe swirls onto tray (star tip).
5. Chill 20–30 mins.
6. Bake 350°F (175°C), 10–12 mins until edges lightly golden.
7. Cool → store airtight.
