Puree the raspberries in a food processor fitted with a steel blade, then pass them through a fine strainer. Or pass them through a food mill. Set aside.
 
In a medium-size heavy saucepan, whisk together the cocoa and heavy cream. Add the butter, sugar, corn syrup, and raspberries and stir until well blended. Place the pan over medium heat and slowly bring the mixture to a boil, stirring often. Once it reaches a boil, let it continue to boil slowly for 8 minutes without stirring. Remove the pan from the heat and pour the sauce into a container.
 
Let it cool for 15 minutes if serving hot, or cover and refrigerate until needed. It will last for at least 1 month. The sauce may be reheated slowly. Makes 2½ cups.