Instructions
For the biscuits: Preheat your oven to 200C (180C fan assisted) or Gas Mark 6. Line two large baking trays with baking parchment paper. Spread the hazelnuts on one lined baking tray. Roast them in the preheated oven for 6 to 8 minutes. Watch them closely to prevent burning. They should be golden-brown. Remove the tray from the oven. Transfer the nuts to a cutting board and let them cool completely. Place the butter and 150g caster sugar in a large mixing bowl. Beat them with a wooden spoon until the mixture is light and creamy. Roughly chop the cooled hazelnuts. Add the chopped hazelnuts, grated lemon zest, plain flour, and 0.5 tsp baking powder to the butter and sugar mixture. Stir everything well until a dough forms. You may need to use your hands to bring the dough together into a ball. Divide the dough into 24 equal pieces. Roll each piece into a small ball. Place these balls on the prepared baking trays. Leave space between each ball for spreading. Gently press each ball to flatten it into a disc, about 1 cm (0.5 inch) thick. Bake the biscuits one tray at a time for 12 minutes. They should be very pale golden-brown. Remove the trays from the oven. Let the biscuits cool completely on the trays. They will be soft at first, but will become crisp as they cool. Store the cooled biscuits in an airtight container. Eat them within five days. For the fool: Rinse the 600g blackberries in a colander. Place the rinsed blackberries in a non-stick saucepan. Sprinkle 75g sugar over them. Stir in 1 tablespoon of lemon juice. Heat the mixture gently for 2 minutes. The blackberries should start to soften and release their juices. Take the saucepan off the heat. Remove and set aside 12 whole blackberries for decorating later. Continue cooking the remaining blackberries. Return the saucepan to gentle heat. Simmer the remaining blackberries for 15 minutes. Stir regularly. They should become very soft. Remove from heat. Press the cooked blackberries and their juice through a fine sieve into a bowl. Use the back of a ladle to push through as much purée as possible. Discard the seeds left in the sieve. Let the purée cool completely. You should have about 325ml (11 fl oz) of purée. In a separate large bowl, combine the 300ml double cream and 100ml yogurt. Whip them with an electric whisk. Continue whipping until soft peaks form when you lift the whisk. Do not over-whip, as the fruit purée will thicken it further. Once the blackberry purée is completely cold, taste it. Add more 2 tablespoons of caster sugar if you prefer a sweeter taste. Pour the cold purée into the bowl with the whipped cream and yogurt. Stir gently, just once or twice. The mixture should look lightly marbled, not fully blended. Spoon the blackberry fool into individual wide glass dishes or one large serving bowl. Avoid over-stirring to keep the marbled look. Garnish each serving with a few small mint leaves and the reserved blackberries. You can sprinkle a little extra sugar on top if desired. Serve immediately with the hazelnut biscuits.