1.Grease a 19cm x 29cm slice pan and line with baking paper, extending paper 5cm over long sides.
2.Stir sugar, milk, glucose and butter in a medium heavy-based saucepan over heat, without boiling, until sugar dissolves. Bring to the boil. Boil, stirring, about 4 minutes or until mixture becomes the colour of creamed honey and begins to catch on the base of the pan.
3.Remove pan from heat and quickly stir in marshmallows and 250g of the chocolate. Flavour mixture with oil or essence to taste. Spread mixture into pan.
4.Working quickly, melt remaining chocolate in small heatproof bowl over small saucepan of simmering water. Spread chocolate over slice. Refrigerate choc-mint slice until set before cutting.