Put the chopped chocolate in a heatproof medium bowl. In a small saucepan, heat the cream with the butter, corn syrup and salt until hot but not boiling. Immediately pour the mixture over the chocolate. Let stand for 5 minutes, then whisk until smooth. Let the frosting cool, whisking occasionally, until thick enough for dipping and glazing the cupcakes, about 30 minutes. For a frosting firm enough to spread, let stand for 1 hour.