In a heavy-bottomed saucepan, heat cream and butter over medium heat. Whisk often to ensure it doesn't burn on the bottom. Once butter is melted, reduce heat to low.

Stir in cocoa powder and sugar, 1/2 cup at a time. Whisk well after each addition to prevent clumping. When the mixture is completely smooth, remove from heat. If you desire a thicker fudge, cook while whisking for several minutes longer). Stir in vanilla extract.

Transfer the fudge to a glass jar or storage container and let cool to room temperature before covering with a lid (the steam will cause condensation to form). Keep it refrigerated until ready to serve. May microwave it or heat on a stovetop.