Butter a 20cm shallow square cake tin. Fill a shallow roasting tin with cold water.
Place the sugar in a heavy-based (not non-stick) medium saucepan with the milk. Heat very gently, stirring occasionally, until the sugar is dissolved. Do not let mixture boil during this time or it will crystallise.
Bring to the boil and leave it boiling for about 20 minutes or until it registers 118 degrees C on a sugar thermometer (i.e. the hard ball stage - when a little of the mixture will form a hard ball when dipped in cold water).
Immediately stand the base of the pan in the cold water and stir in the salt and vanilla essence.
Remove the saucepan from the water and beat the mixture as it begins to go grainy around the edge of the pan. Do not overbeat, otherwise the mixture will not pour; if underbeaten, it will be chewy.
Stir in the chocolate and mix well.
While still pourable, transfer the mixture into the prepared tin and allow to cool slightly until set. Mark the squares and leave until completely cold, then cut into squares and store in an airtight container.