Melt the butter in a heavy-based saucepan, then add the sugar, syrup, condensed milk and milk. Heat very gently until all the ingredients have dissolved. Then bring to the boil, stirring constantly. Continue to heat, stirring gently until the mixture has reached 115C/240F on a sugar thermometer (or until a teaspoonful of the mixture forms a soft ball when dropped into cold water).
Remove from the heat and add the vanilla essence. Beat well until thick and creamy. Pour into shallow greased tins (pans) and mark into squares with a knife. Leave to cool and set. Cut into pieces and store in an airtight container.