Preheat the oven to 150°C and grease a 30x20cm baking tin.
Whisk butter and sugar until fluffy and combine with hazelnuts and flour to make dough.
Press dough into the tin, bake for 30 minutes, and cut into 2.5cm squares while warm.
Increase oven temperature to 180°C and grease another 30x20cm tin.
Melt chocolate and butter, then mix with whisked eggs, sugars, dry ingredients, and pour into the tin.
Bake the brownie for 35 minutes, cool completely, and cut into 2.5cm squares.
Melt milk chocolate with 200ml water and whisk over iced water until whipped cream texture is achieved.
Melt 275g dark chocolate, stir in 25g grated chocolate, pour into a tin, and chill.
Cut the chilled dark chocolate sheet into 2.5cm squares using a knife dipped in hot water.
Assemble by layering biscuit, Chantilly, dark chocolate square, and finally the brownie.