Preheat oven to 200°C and line baking trays with parchment.
Roast hazelnuts for 6-8 minutes until golden-brown and leave to cool.
Beat butter and 150g caster sugar until light and creamy.
Roughly chop nuts and stir into butter with lemon zest, flour, and baking powder.
Form 24 balls of dough and place them spaced apart on trays.
Flatten biscuits to 1cm thick and bake for 12 minutes until pale golden.
Combine blackberries and preserving sugar in a saucepan with lemon juice.
Heat for two minutes until softening and reserve 12 berries for decoration.
Simmer remaining berries for 15 minutes until very soft.
Press berries through a sieve to create a smooth purée and let cool.
Whip cream and yoghurt until soft peaks form.
Gently fold the cold purée into the cream to create a marbled effect.
Spoon into dishes and garnish with mint and reserved berries.