Combine cocoa powder, sugar, corn syrup, light cream, salt and butter in a small saucepan, and cook over medium heat, stirring constantly, until mixture comes to a full boil.

Boil briskly for 3 minutes, stirring occasionally, then remove from heat, and stir in vanilla.

The sauce may be stored, refrigerated in an airtight, heatproof container for up to 1 week.

To reheat, place the container of sauce in a pan of hot, but not boiling, water until the sauce warms and thins to a pouring consistency.