For praline, preheat oven to 170C. Scatter hazelnuts over a tray and roast until golden (6-8 minutes). Set aside to cool, rub with a tea towel to remove skins and set nuts aside. Meanwhile, combine sugar and 60ml water in a saucepan over medium heat, stirring to dissolve sugar. Brush down sides of pan with a wet pastry brush to remove sugar crystals, then cook without stirring until caramel (10-15 minutes). Add hazelnuts, stir with an oiled fork to combine, then spread over a lightly oiled tray and set aside until firm (20 minutes). Break into coarse pieces, transfer to a food processor and pulse until coarsely chopped.
Place dark chocolate (50% cocoa solids) in a bowl. Bring cream to the boil in a saucepan and pour over chocolate. Stand for 2 minutes to melt, then add butter and stir until smooth and glossy. Transfer half the mixture to a separate bowl, add brandy, stir to combine> and refrigerate until firm (1-2 hours). Stir praline through remaining chocolate and refrigerate until firm (1-2 hours).
Meanwhile, for white chocolate ganache, place white chocolate in a bowl. Bring cream to the boil in a small saucepan and pour over white chocolate. Stand for 2 minutes to melt, then add butter and stir until glossy and combined. Refrigerate until firm (1-2 hours).
Roll tablespoonfuls of each of the three mixtures, place on a tray, refrigerate until firm (1-2 hours). Melt remaining chocolate (70% cocoa solids) in a heatproof bowl over simmering water, set aside to cool slightly. Dip truffles in chocolate, dust with cocoa, refrigerate until set (1-2 hours), then serve.