Chocolate Hazelnut Cake with Praline Chocolate Crunch

Line 2 rimmed baking sheets with parchment paper. Stir sugar and 1/3 cup water in a small saucepan over medium-low heat until sugar dissolves. Increase heat and cook without stirring, occasionally swirling pan and brushing down sides with a wet pastry brush, until amber in color, 7-8 minutes. Stir in hazelnuts, then pour onto 1 prepared sheet, separating nuts. Let cool. Transfer candied nuts to a food processor. Add hazelnut oil and purée until smooth to make praline paste. Combine both chocolates and 3 tablespoons praline paste in a medium metal bowl (reserve remaining praline paste for praline mousse layer). Set bowl over a saucepan of simmering water and stir until chocolate is melted and mixture is smooth. Remove from heat and stir in salt, then cereal. Spread out in a thin layer (about 1/8") on second prepared baking sheet. Chill until set, about 30 minutes. DO AHEAD: Can be made 1 day ahead. Cover and keep chilled. Cover and chill remaining praline paste.