Preparation

• Mix the sugar and 1/3 cup cold water in a heavy-duty 3-quart saucepan. Cover and bring to a boil over medium-high heat and cook, covered, checking and swirling occasionally, until the mixture starts to color, 5 to 8 minutes. Remove the lid and swirl more frequently until it is dark amber; it will darken quickly. Do not stir.
• Reduce the heat to low and carefully whisk in the butter. It will bubble up. Keep whisking until the bubbles settle down, then add the cream, whisking until the caramel is smooth, about 2 minutes.
• Remove from the heat and whisk in the rum, salt, and vanilla. Pour the caramel sauce into a heatproof container and cool to room temperature, stirring occasionally.