In a microwave-safe cup, combine the cream and corn syrup. In a medium, wide saucepan, spread the sugar in an even layer. Cook over moderate heat, without stirring, until it begins to caramelize. Swirl the pan and stir to incorporate the caramelized sugar. At this point, microwave the cream mixture at high power for 2 minutes. Continue cooking the sugar until a medium-amber caramel forms, about 4 minutes. Remove from the heat and carefully add the hot cream, stirring with a long-handled spoon. Return the caramel to low heat and add the butter and salt, stirring until any hardened caramel is melted. Transfer the caramel to a heatproof glass jar and let cool.