Make the ice cream. Line a tray with baking paper.
Put the sugar and water in a small saucepan and stir over high heat until it starts melting. Stop stirring and swirl the pan until sugar is completely melted, bubbling, and turning a golden amber colour.
Add the nuts and stir until they are covered in caramel.
Pour the praline on the prepared tray and let it cool completely. Break into pieces and blittz in a food processor with 1 tsp sugar until finely ground.
Whip the double cream until you have stiff peaks. Pour the condensed milk over the whipped cream and use a spatula to fold together.
Fold in the melted white chocolate and ground praline. Transfer to a metal bowl or loaf tin and freeze overnight. Take out of the fridge 10 minutes before serving.
Preheat the oven to 180C (350F) and roast the hazelnuts for about 10 minutes, tossing them now and again so that they don't burn. Cool slightly then blitz in a food processor together with 1 tbsp of the sugar until finely ground.
Lower the heat to 160C (325F) and line the bottom of a 20cm (8in) springform cake tin with baking paper, then spray the tin with cake release.
Place the egg whites, half the sugar and salt in the bowl of your stand mixer.
Whisk until you have stiff peaks. Transfer to a bowl and set aside.
Put the chocolate in a bowl and suspend the bowl over a pot of barely simmering water. Allow to slowly melt, stirring occasionally.
Place half of the sugar and butter in the bowl your stand mixer (no need to clean) fitted with a paddle attachment and beat until light and fluffy, stopping the mixer and scraping the bottom and sides of the bowl as needed.
Add the eggs yolks, one at a time, and beat them in until they are incorporated, again scraping the bowl as needed.
Gradually add the melted chocolate then fold in the nuts and vanilla extract.
Add 1/3 of the egg whites into the chocolate mixture to loosen it then fold in the rest gently trying not to knock the air out.
Pour the batter into the prepared tin and smooth the top. Bake for 50-55 or up to an hour. The cake will rise in the tin and a skewer inserted in the centre will come out with a few crumbs attached.
Cool in the tin then remove and carefully transfer to a cake stand (or plate). Be gentle as the torte is a little fragile - you may want to leave the bottom of the springform tin on.
The cake will settle slightly as it cools and the top may crack a little - this is normal.
Dust the cake with icing sugar and serve with a scoop of the ice cream and a few chocolate shavings.