Make caramel sauce: In a saucepan, combine sugar and water over medium heat. Cook without stirring until deep amber. Remove from heat, whisk in butter, cream, vanilla, and salt. Let cool slightly.
Assemble sundae: Place scoops of vanilla ice cream in a bowl or sundae glass.
Top it: Drizzle with warm caramel sauce. Add whipped cream, nuts, and a cherry.
Serve immediately (and prepare for happy faces!).
Notes
Make caramel sauce ahead and store in fridge up to 1 week.
Warm sauce gently in microwave or stovetop before serving.