Grease an 20 cm/8-inch square dish with oil or butter, line with parchment paper and set aside.
Combine milk, sugar and cocoa in a heavy bottomed, medium sized saucepan and bring to a boil while stirring. Once it starts to boil, reduce to simmer on lowest heat and do not stir again.
Measure the temperature of the mixture using a cooking thermometer and as soon as it reaches 114°C/238°F, remove from the heat and leave to cool to 46°C/115°F.
Add butter and vanilla to the mixture and beat till fully incorporated and the fudge has thickened but is still pourable.
56 grams butter, ½ teaspoon vanilla
Pour into the lined dish and cool in the fridge before slicing and serving.
Notes
Grease a 20 cm/8 inch pan with oil or butter, and line with parchment paper.
Use a heavy bottomed medium saucepan for even heat distribution, and a wooden spoon for stirring.
Combine the cocoa powder, sugar and milk in the pan before turning on heat to medium.
Bring to a boil while stirring the mixture.
Once it starts to boil, DO NOT stir again. Turn the heat down to the lowest setting and allow to simmer (uncovered).
Use a cooking thermometer to measure the heat in the middle of the mixture. My favourite thermometer is the Thermapen (affiliate link) as it is ergonomic, fast and accurate.
If you do not have a cooking thermometer, you could use the soft ball method - drop a tiny amount of fudge into a glass of water and if it rolls into a ball, it has finished cooking.
When the mixture reaches 114℃/238℉, turn off the heat. It normally takes 30-35 minutes to reach this temperature, although different factors might affect it such as type of pan, the heat and altitude.
Leave the mixture to cool to 46℃/115℉, then add the butter and vanilla and using a wooden spoon, stir quickly. At this temperature the fudge will be warm and the crystals formed around the edges will have hardened.
The stirring process prevents sugar crystals from reforming and becoming grainy. A longer beating gives a chewy fudge, while a shorter beating it will be more crumbly.
Pour into the greased tray, cover with plastic wrap and chill in the fridge for 20 minutes.
Remove from the fridge and cut the fudge into squares or rectangles. Peel the fudge away from the parchment paper and serve.
Storing: Fudge may dry out if stored in the fridge after a few days. As long as your kitchen is not too hot, it is best to keep in an sealed container at room temperature and it will remain fresh for up to two weeks.
This chocolate fudge can be frozen for up to a month. To freeze as a block, wrap it well to prevent it from losing moisture and drying out. If you require smaller quantities at a time, cut into small pieces and wrap individually or arrange in layers in a container with sheets of parchment paper in between.