Grease an 8 inch square pan, or line pan with heavy-duty foil and grease foil.
In a mid-sized heavy saucepan over a low heat, melt the chocolate with the milk.
Add in the sugar, corn syrup and salt.
Stir continuously until mixture is boiling.
Reduce heat and continue to cook - without stirring - until mixture reaches 230* F on a candy thermometer. (If you don't have a candy thermometer, you will know your mixture is done when a teaspoonful of the mixture forms a soft ball when dropped into cold water.).
Remove from heat as soon as mixture has reached desired temperature.
Add in the vanilla essence and the butter, but do not stir.
Allow mixture to cool in saucepan until it reaches 110* F on your candy thermometer. (If you don't have a candy thermometer, cool mixture until it is lukewarm.).
Once mixture has reached desired temperature, beat it with a wooden spoon until the mixture begins to lose its gloss and starts to thicken (approximately 15 minutes).
Pour into pan. Allow mixture to stand until cool and hard.
Turn fudge out of the pan and onto a cutting board and cut into squares.