Place egg whites in the bowl of a heavy-duty stand mixer; let stand at room temperature for 30 minutes.
In a microwave-safe 2-qt. bowl, combine the sugar, water, corn syrup and salt. Microwave, uncovered, on high for 4 minutes; stir until sugar is dissolved. Cook 6-8 minutes longer or until a candy thermometer reads 260°(hard-ball stage), stirring every 2 minutes. Just before the temperature is reached, beat egg whites on medium speed until stiff peaks form.
With mixer running on high speed, carefully pour hot syrup in a slow, steady stream into the bowl. Beat just until candy loses its gloss and holds its shape, 15-20 minutes. Immediately stir in pecans and vanilla. (Do not overmix, or candy will get stiff and crumbly.)
Working quickly, drop by tablespoonfuls onto waxed paper. Let stand at room temperature overnight or until dry to the touch. Store in an airtight container at room temperature.
Editor’s Note: 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.