For bitter chocolate sorbet, stir ingredients and 400ml water in a saucepan over medium-high heat until melted and smooth, then bring to the boil. Remove from heat, cool to room temperature, then refrigerate to chill. Churn in an ice-cream machine, then freeze until required. Makes about 1 litre.
For praline mousse, stir sugar and 50ml water in a saucepan over medium-high heat until sugar dissolves, then bring to the boil and cook, without stirring but brushing down sides of pan with a wet pastry brush if sugar crystals start to form, until dark caramel (4-5 minutes). Remove from heat, add hazelnuts and stir quickly to combine, then tip out onto an oiled baking tray and set aside until it sets hard (30-40 minutes). Break into shards, then process two-thirds of the praline in a food processor with oil until a paste forms (reserve remaining praline to serve; store in an airtight container). Combine praline paste in a bowl with chocolate. Bring liqueur and half the cream to the boil in a saucepan over medium-high heat, then pour into chocolate mixture and stir until smooth and combined. Whisk remaining cream in a separate bowl to soft peaks, fold into praline mixture and refrigerate until firm (2-3 hours; this can be done 2-3 days ahead).
Preheat oven to 150C. Bring cream, milk and vanilla to the simmer in a saucepan over medium-high heat. Whisk yolks, egg and sugar in a bowl to combine, pour cream mixture over, whisk to combine and set aside.
Spread half the bread slices with a little praline mousse, then sandwich with remaining slices of bread. Cut into triangles, then layer in a 2-litre ovenproof dish, scattering with bitter chocolate as you go. Pour cream mixture over bread, allow to soak for 15 minutes, then scatter some of the praline shards over and a little demerara sugar and bake until golden and set (45-50 minutes). Stand for 15 minutes, then dust with icing sugar and serve warm with chocolate sorbet and extra praline shards.