Make sure to prepare a pan or a tray to put your fudge mixture in. Spray the pan with some butter or a cooking spray then you can set it aside.
Put the sugar, milk, and cocoa in a pot or a saucepan and combine them. Let the mixture boil while constantly stirring it.
Reduce the heat and let it simmer. There is no need to stir the mixture in this step.
Use a thermometer to check and ensure that the temperature is 238°F(114°C).
Take the saucepan from the heat and add butter and vanilla. Stir the mixture with a wooden spoon until it loses its glossiness. Make sure to not overbeat it.
Transfer the mixture to the tray and let it cool down.