Line two baking sheets pans with waxed paper; set aside.
In a medium saucepan, combine sugar, corn syrup, and 1/2 cup water. Bring to a boil, stirring just until sugar dissolves, and cook until syrup reaches 250 degrees on a candy thermometer.
Meanwhile, in the bowl of a stand mixer fitted with the whisk attachment, starting on low and gradually increasing to high, beat egg whites with salt until stiff peaks form. If peaks form before the syrup reaches 250 degrees, reduce speed and keep mixer running on low.
With the mixer on medium speed, add the syrup in a slow, steady stream (taking care not to hit the whisk or side of the bowl). Add vanilla. Increase speed to high and continue to whisk until mixture holds its shape and begins to lose its gloss, about 15 minutes.
Prepare a small bowl of hot water for dipping and a 1-tablespoon cookie scoop OR two spoons.
Working quickly, drop 1 heaping tablespoon candy onto prepared sheet pans, dipping scoop OR spoons into hot water as needed to prevent sticking. Gently press a pecan half into the center of each candy. Let candy cool completely, at least 4 hours and up to overnight. Candy can be stored in an airtight container at room temperature up to 1 week.
Cook’s Note: It’s best to make the divinity on a dry day. Too much humidity prevents it from setting. So if it’s raining, choose another day to make it!
For best texture, work quickly when scooping the candy. If it's setting too quickly, stir in a few drops of hot water until smooth.