Remove the seeds from the vanilla bean by scoring down the length and scraping the seeds out of each half.
Whisk the vanilla seeds and sugar with the egg yolks in a large bowl until pale. In a second bowl whisk the cream until soft peaks form. (Important! Please don't overwhip it.) Then in the third bowl whisk or beat the egg whites with an electric mixer with a pinch of salt until they form very firm peaks (this is when you can pull the egg whites in any direction and they will stay like it).
At this point add the praline, the cream and egg whites, to the egg yolk mixture. Gently fold in.
Immediately scoop the contents into your chosen container. Cover wtih plastic wrap and freeze until you're ready to eat it.
Roast the hazelnuts in the oven at 225C/425F/Gas 7 until golden (about 4 minutes). Really watch them, because if you over-roast them they go bitter and you can't use them.
Put the sugar and water in a thick-bottomed pan and place on a medium to high heat. The mixture will start to bubble and then turn into a clear syrup.
To begin with, it will gradually start to color in parts or from the sides. Gently and carefully shake the pan, just moving it to mix the patches of color.
When it's all golden brown, carefully tip it away from you and gently add the nuts. Turn the heat down to a simmer and gently stir to coat the nuts in caramel.
When the caramel is dark golden brown, turn it out on to a clean, lightly oiled tray, or on to greaseproof paper on a surface that won't burn. It will cool to a flattish solid sheet.
When completely cooled (which takes about 20 minutes), smash it up roughly and pulse it in a food processor until the pieces are still quite chunky (very approximate size about 1/2 cm/ 1/4-inch).
Remove about half the praline, then pulse the rest to a powder (or put it in a tea-towel and bash with a rolling-pin), and add both lots of praline to the semi-freddo mixture.