Preheat oven to 180C. Roast hazelnuts on a baking tray until golden (4-5 minutes), rub with a tea towel to remove skins, place on a lightly oiled oven tray. Combine sugar and 80ml water in a small saucepan, stir over medium-high heat until sugar dissolves, bring to the boil and cook until dark caramel (4-5 minutes). Pour onto hazelnuts, cool completely, break into pieces and process in a food processor until finely chopped. Bring cream to the simmer in a saucepan over medium-high heat, add chocolates, stir until smooth. Remove from heat, cool slightly, stir in praline and refrigerate until firm (1-2 hours).
Thickly spread half the brioche slices with chocolate mixture, sandwich with remaining slices. Whisk eggs and milk in a bowl, transfer to a shallow tray. Stand sandwiches in egg mixture, turning once, until soaked (2 minutes each side), then shake off excess.
Heat one-third of butter in a large frying pan over medium-high heat until foaming, add 2 sandwiches and cook, turning once, until golden (2-3 minutes each side), transfer to a baking tray. Repeat with remaining butter and sandwiches, wiping pan clean between batches. Warm in oven for 5 minutes, then serve dusted with icing sugar.