Mrs. Milman's Chocolate Frosting

Put chocolate and cream into a medium saucepan. Cook over medium-low heat; stirring constantly with a heatproof rubber spatula, until thick, 30 to 35 minutes. Remove from heat. Stir in corn syrup. Transfer to a large, wide metal bowl. Refrigerate until frosting is cool enough to spread, about 2 hours, checking and stirring every 15 minutes. Use immediately.