To make the praline, grease a cool surface (such as a baking tray or marble pastry board) and grease a palette knife with oil or butter. Next, make a caramel. Put the sugar into a heavy-based pan and place over a moderate heat.
Shake the sugar around gently so that it heats through evenly. Don't stir or the sugar will crystallise as it melts. (Caution: making caramel can be dangerous because melted sugar is very hot. Take care and don't leave the caramel unattended.)
After a few minutes the sugar will begin to melt in patches here and there. It will swiftly begin to colour. Keep swirling and turning the pan gently so that the sugar melts to a syrup evenly. Ensure that all of the sugar crystals have melted.
The sugar will turn a buttery colour, then a honey colour. Take the pan off the heat as soon as the syrup has turned a light hazelnut brown and immediately tip in the almonds. Quickly stir the nuts and the syrup together.
Very quickly pour the nut mixture out onto the prepared surface. Smooth the praline down quickly but gently with the buttered palette knife, then leave to cool completely, about 30 minutes.
When completely cool, chop the praline up into small pieces with a heavy knife.
For the semifreddo, heat the maple syrup to boiling point in a heavy-bottomed pan over a high heat.
Meanwhile, place the egg white in a clean bowl with the pinch of salt. Whisk the egg white with an electric whisk until it forms firm peaks. When the maple syrup has reached boiling point, slowly pour the hot syrup into the egg white, whisking constantly.
Continue whisking until you have a thick, stiff meringue, for 5-10 minutes or so. The whisk will form a pattern in the mixture when it's ready. Leave the mixture to cool, about 20 minutes.
In another clean bowl, whisk the cream until it just holds its shape when the whisk is removed, then fold it into the maple meringue mixture with a metal spoon. Finally, fold in the chopped praline.
Spoon the mixture into the cling film-lined dariole moulds or loaf tin. Shake them gently to expel any pockets of air. Place the moulds or loaf tin into the freezer and freeze for 3-4 hours or overnight, until solid. There's no need to stir the semifreddo as it freezes because the whisking and the cream ensure a smooth, creamy texture.
Transfer the semifreddo to the fridge to soften 20 minutes before serving. Turn out the dariole moulds onto individual serving plates and remove the cling film. If using a loaf tin, turn the semifreddo out onto a plate, remove the cling film and cut into slices. Drizzle with maple syrup and serve.