1.Grease 20cm x 30cm (8-inch x 12-inch) rectangular pan; line base and long sides with baking paper, extending paper 5cm (2 inches) over sides.
2.Melt 185g (6 ounces) of the dark chocolate; stir in half the rind. Spread chocolate over base of pan; refrigerate until set.
3.Meanwhile, stir sugars, syrup, cream, milk and another 200g (6½ ounces) of the dark chocolate in medium saucepan over heat, without boiling, until sugar dissolves. Bring to the boil; boil, without stirring, about 10 minutes or until mixture reaches 116°C/235°F on a candy thermometer. Remove from heat, leaving thermometer in mixture; add butter, do not stir.
4.Cool fudge up to 1½ hours or until the temperature of the mixture drops to 40°C/80°F. Remove thermometer. Stir fudge with wooden spoon about 10 minutes or until a small amount dropped from the spoon holds its shape. Spread fudge over chocolate in pan; smooth surface. Cover with foil; stand at room temperature about 2 hours or until set.
5.Melt remaining dark chocolate; stir in remaining rind. Melt white chocolate; tint orange. Working quickly, spread dark chocolate over slice; drizzle with orange chocolate. Pull a skewer through chocolate mixture for a feather and fan effect. Stand at room temperature until chocolate sets before cutting.
Glucose syrup is also called liquid glucose; it’s available in most supermarkets and delicatessens. Store fudge in an airtight container for up to a week. Slice is best served at room temperature. Using a candy thermometer will ensure perfect results.