Preheat oven to 180C / 350F / 160C fan forced. Grease the sides of a 22cm (9 inch) round springform cake tin and then line the bottom and sides with baking paper.
Line a baking tray with baking paper, then tip the hazelnuts onto the tray in one, even layer. Toast in the oven, shaking the pan every 5 minutes or so until golden all over. Allow the hazelnuts to cool for 5 minutes, then grind them to very fine crumbs in a food processor. Keep an eye on them as if you blend too long, they will release their oil and become wet instead of the dry ground hazelnuts you need.
Beat the butter and sugar together until light and creamy, then add the vanilla and the yolks, one at time until fully combined and the mixture looks fluffy. Finally add the melted chocolate and mix through.
Mix the ground hazelnuts, cocoa, baking powder and salt together. With the beater on low, slowly pour it into the butter mixture until combined.
In a clean bowl, whisk the egg whites until soft peak stage. Add 1/3 of the egg whites to the batter and mix through. Add the last 2/3 in 2 parts but just fold through in gentle, measured folds until all combined. Keep the mixing to a minimum to keep as much air in the mix as possible.
Bake for 50 - 55 minutes until the top looks dry and a toothpick comes out with just a moist crumb or two.
Allow to cool for half an hour before removing the springform tin. Transfer to a cake plate and allow to cool completely.
Dust with cocoa right before serving
I use a standard Australian 20ml tablespoon (4 teaspoons worldwide)
For best results you should always weigh ingredients like flour and sugar. Kitchen scales like these are relatively cheap but if you can’t weigh the ingredients, use the spoon and level method (don’t scoop).