Preheat the oven to 180°C/fan 170°C/350°F/gas mark 4. Butter well a 20cm round cake tin, and line the base with baking parchment.
Roast the hazelnuts in the oven for 5-10 minutes, watching carefully so they don’t bum, then cool and grind finely in a food processor. Place the chocolate, 70g of the sugar and the butter in a bowl set over very gently simmering water (ensure the base of the bowl does not touch the water) and melt together gently. Remove from the heat and stir in the hazelnuts. Beat the egg yolks together until they change to a paler colour, then mix them into the cooled chocolate mixture.
In another very clean bowl, whisk the egg whites, then slowly add the remaining caster sugar until the mixture forms soft peaks. Take a large spoonful and fold it into the chocolate to lighten it a little. Then fold in all the remaining egg whites, as gently and lightly as you can, using a large metal spoon or spatula. Transfer the batter to the tin and bake for about 20 minutes. This cake is fragile, so take care when handling it. Leave to cool for 10-15 minutes, then run a knife around the edge to loosen it from the tin. When the cake is cold, turn out very carefully on to a serving plate. The base will become the top.
To make the ganache, melt the chocolate, cream and butter in a bowl set over very gently simmering water (make sure the base of the bowl does not touch the water). Stir together, cool a little until it thickens, then pour it over the cake. It will be quite easy to spread with a palette knife. Stud the surface with the raspberries.