Preheat the oven to 350°. Generously oil a rimmed baking sheet. In a bowl, toss the crisped rice and peanuts with the corn syrup until evenly coated; add a pinch of salt. Spread the mixture as thinly as possible on the baking sheet and bake for 10 minutes, until crisp. While still warm, transfer the crunch to a plate; let cool. Break the crunch into large pieces.
Scoop vanilla ice cream into sundae glasses and pour hot chocolate sauce on top. Top with the crunch and serve.
Make Ahead: The crunch can be stored overnight in an airtight container.