In a medium saucepan over medium-low heat, melt butter completely.
Add brown sugar and stir just a little. Then, let sit until partially melted, about 3 minutes. Keep the stirring to a minimum to avoid crystal formation.
Add heavy whipping cream and stir once to completely incorporate, then bring to a boil until the mixture reaches 225 F on a candy thermometer (about 5 minutes).
Remove from heat and stir in vanilla extract and salt.
Cool, store in an airtight container, and refrigerate for up to three weeks or freeze for up to three months.
