Place egg whites in the bowl of a stand mixer; let stand at room temperature for 30 minutes. Meanwhile, line three 15x10x1-in. pans with waxed paper.
In a large heavy saucepan, combine the sugar, water and corn syrup; bring to a boil, stirring constantly to dissolve sugar. Cook, without stirring, over medium heat until a candy thermometer reads 252° (hard-ball stage). Just before the temperature is reached, beat egg whites on medium speed until stiff peaks form.
Slowly add hot sugar mixture in a thin stream over egg whites, beating constantly and scraping sides of bowl occasionally. Add vanilla. Beat until candy holds its shape, 5-6 minutes. (Do not overmix or candy will get stiff and crumbly.) Immediately fold in pecans.
Quickly drop by heaping teaspoonfuls onto prepared pans. Let stand at room temperature until dry to the touch. Store between waxed paper in an airtight container at room temperature.
We recommend that you test your candy thermometer before each use by bringing water to a boil; the thermometer should read 212°. Adjust your recipe temperature up or down based on your test.