Combine the light corn syrup, sugar, salt, and water in a large saucepan, stirring to moisten the sugar evenly. The mixture should have no dry pockets before heating.
Heat the mixture while moving a spoon steadily through it until the sugar has dissolved. Look for a smooth syrup without visible sugar granules.
Stop stirring and let the syrup cook to 250°F on a candy thermometer. Watch the reading with the sensing tip clear of the saucepan bottom.
Use the stand mixer to whip the egg whites into a firm foam. When you lift the whisk, the peaks should stand upright rather than fold over.
With the mixer beating continuously, feed approximately half of the hot syrup into the whites in a narrow, gradual stream. Aim along the inside of the bowl, clear of the moving whisk, to limit splashing.
Return the saucepan containing the remaining syrup to the burner and heat it to 270°F. The syrup will become more concentrated as it cooks.
Keep the mixer running as you gradually pour the remaining hot syrup into the whipped mixture. Use the same controlled stream, avoiding direct contact with the spinning whisk.
Beat the mixture until it becomes thick enough to retain a distinct mound. It should look shiny but substantial, with ribbons that hold rather than immediately disappear.
Distribute the vanilla and chopped pecans through the thickened candy. The nuts should appear evenly scattered throughout the mixture.
Use spoons to portion individual mounds onto parchment, then leave them to cool and set completely. Finished pieces should retain their peaks instead of spreading into puddles.