1. Place vinegar in a large, heatproof bowl.

2. Place butter and cream in a small, heavy-bottomed saucepan and bring to a boil over medium-high heat. Reduce heat to medium-low and whisk constantly until mixture thickens to consistency of mayonnaise and the milk solids separate out and float to the top, about 5 minutes. Transfer mixture to the bowl of a food processor and process for 20 seconds to break up solids. Return mixture to saucepan and cook over medium-low heat, whisking constantly until it becomes a deep brown, about 3 minutes.

3. Add mixture to vinegar (it will foam up). Add salt and swirl bowl to combine; set aside.