Preheat oven to 350 degrees F. Coat 2 baking sheets with cooking spray.Advertisement
Whisk flour, cornstarch, baking powder and salt in a mixing bowl. Beat 1/2 cup sugar, butter and oil in another mixing bowl with an electric mixer on medium speed until fluffy. Add egg white, lemon zest and vanilla; beat until smooth. Beat in lemon juice. Add the dry ingredients to the wet ingredients and fold in with a rubber spatula just until combined.
Drop the dough by teaspoonfuls, 2 inches apart, onto the prepared baking sheets. Place the remaining 1/4 cup sugar in a saucer. Coat the bottom of a wide-bottomed glass with cooking spray and dip it in the sugar. Flatten the dough with the glass into 2 1/2-inch circles, dipping the glass in the sugar each time.
Bake the cookies until they are just starting to brown around the edges, 8 to 10 minutes. Transfer to a flat surface (not a rack) to crisp.
Make Ahead Tip: Store in an airtight container for up to 3 days.
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