Line a baking sheet with a silicone mat and set aside. Heat sugar, water, and corn syrup in a large, wide saucepan over medium-high heat and stir until sugar is dissolved, about 1 minute.

Increase heat to high and cook without stirring until the mixture has turned caramel-brown, about 10 minutes. Remove from heat and stir in butter. When butter has melted, stir in nuts and half the salt.

Pour mixture onto the prepared baking sheet and use a silicone spatula to spread it to about ¼-inch thickness. When brittle has cooled a little, sprinkle cacao nibs and remaining salt over it, pressing to adhere.

Cool completely, about 60 minutes. Break into small chunks and sprinkle over sundaes. The remaining brittle will keep in an airtight container for about 2 weeks.