Mix together the butter, cocoa, cream, and sugar in a small pot.
Place the pot on the stove and warm on medium heat, stirring until everything has been combined and the butter has melted. This should take a few minutes.
Take the contents of the pot and put it into a mason jar. Store the jar in the fridge. Your fudge will get harder as it starts to cool, so when you want to use it again, just put the desired amount in a saucepan and heat on a medium to low heat.