In a medium saucepan, add in the sugar and cocoa powder. Whisk thoroughly to break up any clumps of cocoa powder as these won't break down once cooked and you'll have little clumps throughout your fudge. I like whisking this off the heat before anything else so it is super smooth and fine.

Turn the heat onto medium and add in the heavy whipping cream and butter. Continue whisking on medium heat until mixture melts together and thickens, stirring constantly to prevent scorching. Once mixture is thick and smooth, remove from the heat and whisk in the vanilla extract. Pour the mixture into a large Mason jar and seal. Can be served warm, where it will be more thin like a chocolate syrup, or refrigerated until thicker like a hot fudge. Store in the fridge.