Preheat the oven to 170C. Grease and line an 20cm/8” loose bottomed cake tin.
Place the egg whites in a bowl with one tablespoon of the caster sugar. Set aside.
In a second bowl add the egg yolks, the remainder of the caster and a little vanilla bean paste (optional). Have the hazelnuts blitzed and ready to use as you want to assemble this cake quite quickly.
Melt the butter and chocolate over a low heat until combined. Allow to cool for 5 minutes. Using a hand held electric mixer whisk the yolks and sugar until very pale and fluffy.
Next, with clean whisks, whip up the whites until stiff peaks form. To combine stir the cooled chocolate mixture through the yolks and sugar. Do this gently so as not to knock out too much air. Once even in colour fold through the whites and hazelnuts. Again do this very gently so as not to knock out any more air.
Pour into your prepared tin, scatter with excess hazelnuts, then bake for 40-45 minutes or until just set in the middle. It will sink down so don’t worry if it looks like it has dipped or cracked – adds to the rustic charm!
Allow to cool completely. If you want you can place it in the fridge so you can cut neat fudgy slices, or simply attack with a fork and a generous helping of softly whipped Graham's Double Cream.