Preheat the oven to 180C/350F/Gas 4. Snap the chocolate into pieces in a small heatproof glass bowl. Place the bowl over a small pan of simmering water, with the base of the bowl not quite touching the water. Allow the chocolate to melt. Don't be tempted to stir it, other than to occasionally push any unmelted chocolate down into the liquid chocolate to encourage it to melt. Turn off the heat as soon as the chocolate has melted.
Cream the butter and sugar together in a food processor until smooth and creamy. Break the eggs and vanilla extract in a small bowl or jug, whisk just enough to break up the eggs, then add the mixture gradually to the butter and sugar, beating constantly. It is worth scraping down the sides of the bowl with a rubber spatula from time to time to ensure a thorough mixing. Add the melted chocolate and continue to mix.
Toast the hazelnuts in a shallow pan until golden, shaking regularly so they colour evenly. Grind the nuts coarsely using a pestle and mortar to the texture of gravel, then remove half and continue grinding the other half until it resembles fine breadcrumbs. Add the flour and both textures of nut to the cookie mixture. Stop beating as soon as everything is combined.
Place large, heaped tablespoons of the mixture on to a baking tray lined with baking parchment. You should get twelve large biscuits. The mixture is fine to sit for a few minutes if you are cooking them in two batches. Don't be tempted to flatten the cookies, they will do so in the oven anyway.
Bake the cookies for 10-12 minutes. The cookies will have spread and be very soft to the touch. Remove them from the oven and set aside to cool a little. As soon as they are cool enough to move without breaking, slide a palette knife underneath and carefully lift them onto a cooling rack. Serve with a handful of cherries and raspberries.