Place the chopped chocolates in a medium bowl. In a small saucepan, combine the heavy cream with the sugar and cook over moderate heat, stirring, until the sugar dissolves. Pour the warm cream over the chocolate and let stand until melted, about 5 minutes; whisk until smooth. Whisk in the butter, vanilla and a pinch of salt. Let the dark chocolate glaze cool until slightly thickened, about 15 minutes.