In a large microwave-safe bowl: Combine sugar, cocoa and heavy cream. Cook in the microwave on high for 2-3 minutes until the sugar dissolves and the mixtures boils.
Stir in the 4 tablespoons of butter and cook on high 2-4 minutes more. Please watch carefully and stir every 30 seconds until the mixture thickens. Stir in the vanilla and a pinch of salt.
