Heat oven to 325°F; parchment-line 9-by-13-inch metal pan with lifting overhang.

Beat butter and 1/2 cup sugar 1 minute until smooth without much air. Mix in vanilla, then flour, cornstarch, and salt to crumbly clumps.

Press evenly into pan; fork-prick all over. Lightly knife-mark 24 fingers; scatter tablespoon sugar over.

Bake 30 to 35 minutes until surface dry and edges pale gold. Stand 5 minutes; recut marks while warm.

Cool completely in pan; parchment-lift and separate fingers.