In a medium saucepan over medium heat melt the butter and whisk together with the cream, sugar, and salt. Bring to a gentle boil and continue to boil for 5 minutes, stirring occasionally. Remove from heat and stir in vanilla. Taste the sauce and add more vanilla or salt as desired.
Serve the butterscotch sauce hot or cold. To reheat, put it in the microwave for 15-30 seconds. The sauce will keep in the fridge for several weeks.