For the biscuit base, preheat the oven to 150C/300F/Gas 2 and grease a 30x20cm/12x8in baking tin.
Mix the butter and sugar together until pale and fluffy. Add the hazelnuts and flour and knead lightly to combine.
Push the dough into the prepared tin to form an even layer. Bake for approximately 30 minutes, or until just browning at the sides. Remove from the tin immediately and cut the biscuit into 12 x 2½cm/1in squares using a sharp knife. Set aside to cool.
For the brownie layer, turn the oven up to 180C/350F/Gas 4 and grease a 30x20cm/12x8in baking tin.
In a pan set over a low heat, melt the chocolate and butter together. In a separate bowl, whisk the egg and egg yolk together before beating in the sugars. When the chocolate has melted, pour it into the egg mix. Fold in the salt, flour, baking powder and cocoa, then pour the mix into the prepared baking tin.
Bake for 35 minutes, or until the surface has set. Set aside to cool completely before cutting into 2½cm/1in squares.
For the chocolate Chantilly, fill a large bowl with iced water and sit a smaller bowl in it. Melt the chocolate and 200ml/7fl oz water together in a pan set over a low heat. When the chocolate has melted, pour it into the small bowl and whisk with a handheld electric mixer until it has the texture of whipped cream. Transfer to the fridge to chill.
For the chocolate top, slowly melt 275g/9¾oz of the dark chocolate in a heatproof bowl set over a pan of simmering water. Grate the remaining 25g/¾oz of chocolate, and stir into the melted chocolate until thoroughly combined. Remove the chocolate from the heat, and pour into a 30x20cm/12x8in baking tin. Leave to cool and then place in the fridge to chill.
Boil a kettle, and pour the boiling water into a heatproof cup or glass. Stand a small sharp knife in the hot water, and, using the hot knife, cut the chilled chocolate into 2½cm/1in squares.
Assemble the petits fours with a layer of biscuit on the bottom, spread on some chocolate Chantilly, and top with the dark chocolate and finally brownie.