Lay sheets of waxed paper onto your countertopand set aside.
Heat the sugar, corn syrup, and water over medium heat in a heavy-bottomed saucepan. Using a candy thermometer, heat the mixture to 250º F, the hard ball stage.
As the syrup cooks, whip the egg whites with a pinch of salt until stiff peaks form.
When the sugar syrup has reached the hard ball stage (250º F), remove it from the heat and slowly pour it into the beaten egg whites, whisking at high speed.
Add the vanilla and continue whipping the mixture on high until it holds its shape and does not fall back onto itself, about 5-6 minutes. Stir in chopped pecans, if using.
Using two teaspoons and working quickly, drop scoops of divinity onto sheets of waxed paper placed on cooling racks. If the divinity begins to harden, add a few drops of warm water to help loosen it as you work.
Allow the divinity to dry and harden. This usually takes a couple of hours to overnight. Store in an airtight container for up to two weeks.