In a small saucepan, combine sugar and ¼ cup (60 grams) water. 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; add butter and salt, whisking until butter is melted and incorporated. Pour into a heatproof container, and let cool completely before using.