1. Melt the chocolate and butter in a bowl set over a pan of barely simmering water, stirring until smooth. 2. While the chocolate is melting, in a medium saucepan, whisk together the powdered sugar, brown sugar, cocoa powder, corn syrup , salt, and heavy cream or crème fraîche. Heat the mixture, stirring frequently, until it comes to a low boil. Continue to cook, stirring constantly, until the sauce starts to thicken, about 45 seconds. 3. Remove from heat and whisk in the melted chocolate.
