1.Line base and long sides of 10.5cm x 23.5cm (4-inch x 9½-inch) (top measurement) loaf pan or terrine with baking paper or foil, extending paper 5cm (2 inches) over sides.
2.Press barely-softened chocolate gelato into pan; smooth surface. Cover; freeze about 1 hour or until firm.
3.Meanwhile, make honeycomb by lining oven tray with baking paper, combine sugar, syrup and the water in small saucepan; stir over high heat, without boiling, until sugar dissolves. Bring to the boil; boil, uncovered, without stirring, about 8 minutes or until syrup reaches 154°C/309°F (hard crack) on a candy thermometer. (Or, a small spoonful will snap when dropped into a cup of cold water.) Remove from heat, stir in sifted soda; the mixture will foam. Pour mixture onto tray without spreading; stand about 1 hour or until firm. Break honeycomb into pieces.
4.Stir half the honeycomb pieces into barely-softened vanilla gelato; press over chocolate gelato in pan, smooth surface. Cover; freeze about 2 hours or until firm.
5.For chocolate fudge sauce, break chocolate into medium saucepan. Chop butter; add butter and cream to pan, stir over low heat until smooth. Remove from heat; gradually whisk in sifted icing sugar.
6.Turn gelato terrine onto board; peel away lining paper. Cut terrine into eight slices. Serve with remaining honeycomb and warm fudge sauce.
Be aware that the honeycomb will foam up very quickly once the bicarbonate of soda is added. Store the honeycomb in an airtight container in a cool dry place at room temperature for up to 2 days. Candy thermometers are available from kitchenware stores. Digital thermometers are easier to read. The recipe can be made a day ahead; reheat sauce over a low heat before serving. If you have one, use a silicone loaf pan its smooth surface means there’s no need to line it and, with its sharp corners, the terrine is easy to turn out. If you prefer, use either ice-cream or gelato for both layers.