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1. Preheat oven to 350°F.
2. Place sugar, butter, vanilla and lemon zest in the bowl with the kneading/crushing blade. Mix at speed 8 for 1 minute. Scrape the sides and bottom of the bowl at 30 seconds and at end of 1 minute. Add flour, cornstarch, and salt. Mix at speed 8 for 1 minute or until dough just comes together.
3. Pat dough into an 8 x 8-inch baking pan. Poke crust several times with a fork. Sprinkle top with additional sugar, if desired.
4. Bake for 20-22 minutes or until very lightly browned. Cool completely. Cut into squares.