Place chocolate and cream in a heavy saucepan. Cook over low heat, stirring constantly with a rubber spatula, until combined and thickened, 20 to 25 minutes. Increase heat to medium-low, and cook, stirring, 3 minutes more. Remove from heat; stir in corn syrup. Transfer to a large metal bowl; refrigerate. Check every 15 to 20 minutes; stir until cool enough to spread, about 2 hours. Spread on cooled cake.