In a heavy saucepan, bring sugar, corn syrup, water and a pinch of salt to a boil over medium heat, stirring until the sugar is dissolved, then boil the mixture WITHOUT stirring until it turns a golden caramel.
Remove the pan from heat, and carefully add in cream and vanilla (mixture will bubble up and steam).
Stir in pecans and quickly pour mixture over top of cooled brownies, spreading with a spoon.