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, add the vanilla seeds to the cream and whisk the lactose free cream until it just holds its shape when the whisk is removed, then fold it into the maple meringue mixture with a metal spoon.
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.