Use a double boiler to melt the chocolate and cream together. (Make your own version by placing a glass or stainless steel bowl on top of a pot with simmering water). Stir occasionally until the chips are melted.
Off the heat, stir in the corn syrup and vanilla and allow the mixture to cool to room temperature.
In the bowl of an electric mixer fitted with the whisk attachment, whisk the chocolate mixture, softened butter and icing sugar on medium speed for a few minutes, until it has thickened.