Grease a 19cm x 30cm rectangular slice pan. Line the base and sides with baking paper extending 2cm above edges of the pan.
Combine chocolate bars and butter in a bowl and mix well. Press mix evenly over base of prepared pan. Cover and refrigerate while making filling.
To make filling, beat cream cheese and the sugar in a small bowl with an mixer until light and fluffy. Beat in eggs, one at a time then fold in the sour cream an essence. Tint green with food colouring (optional) and pour filling over biscuit base.
To make topping, place chopped chocolate and coffee dissolved in water in a small heatproof bowl. Sit bowl over a pan of simmering water (can be microwaved on low). Stir until chocolate is melted.
Drizzle topping over filling in pan. Using a skewer, lightly swirl the mixtures together.
Cook in a moderate oven (180C) for about 30 minutes or until set. Remove and cool in the slice pan.
Serve slice cut into squares with whipped cream and mint.