"I found this recipe quite some time ago and have turned it into my own. It is utterly delicious. I tend to eat it while it is still a little runny; if you want a firmer texture, freeze overnight. Put in a fancy glass for a great dessert."
Lightly grease a baking sheet with oil. Line a 10-inch springform tin with plastic wrap.
Combine 1 cup sugar and water in a small saucepan over medium-low heat. Cook, without stirring, until sugar dissolves and syrup starts to bubble and turn golden brown, about 8 minutes. Stir in hazelnuts. Remove from heat and pour evenly onto the oiled baking sheet. Let praline cool until solid, about 15 minutes.
Place half of the hazelnut praline in a food processor; blend into a fine powder. Pour into a bowl. Blend remaining praline into coarse chunks.
Beat heavy cream in large bowl with an electric mixer on medium speed until firm peaks form.
Combine egg yolks and remaining 1 cup sugar in another bowl; beat with the electric mixer on medium speed until smooth and pale, 3 to 5 minutes. Fold into whipped cream.
Beat egg whites in another bowl with an electric mixer on medium speed until stiff peaks form. Fold into the whipped cream and egg yolk mixture. Fold in hazelnut powder and hazelnuts chunks. Spoon into the prepared springform tin; cover with plastic wrap.
Freeze until semifreddo hold its shape, about 1 hour. Stir well. Return to freezer and freeze, stirring every hour, until firm, 4 to 5 hours more.