Melt 1 stick of butter in a medium saucepan over medium heat. Whisk in 1 cup each dark brown sugar and heavy cream and 1 teaspoon kosher salt. Bring to a boil, then reduce the heat and simmer, stirring, until thickened, 5 to 10 minutes. Remove from the heat and stir in 1 teaspoon vanilla bean paste. Make sundaes with blondies, rum raisin ice cream, the butterscotch sauce and chopped cashews.