In a small saucepan, stir together sugar and ¼ cup (60 grams) water until combined. Bring to a boil over high heat. (Do not stir.) Cook, without stirring, until caramel turns a deep amber color and an instant-read thermometer registers 355°F (180°C) to 360°F (182°C), 15 to 18 minutes.

Remove from heat. Carefully whisk in cream until well combined. Add butter and salt, whisking until butter is melted and well combined. Stir in vanilla. Pour into a heatproof container, and let cool completely.