Step 2

Beat the butter and sugar together until light and creamy. Roughly chop the cooled hazelnuts and add to the bowl along with the lemon zest, flour and baking powder. Stir until the mixture comes together into a ball, using your hands if needed.

Step 3

Divide the dough into 24 pieces and roll into balls. Place well apart on lined baking sheets and press to about 1cm thick.

Step 4

Bake one tray at a time for 12 minutes until very pale golden-brown. Leave to cool on the trays — the cookies will be soft when hot but crisp as they cool.

Step 5

While the cookies bake, make the fool: rinse the blackberries in a colander. Put in a non-stick saucepan with the 2 3/4oz sugar and lemon juice. Heat gently for 2 minutes until the berries begin to soften.

Step 6

Reserve 12 blackberries for decoration. Continue simmering the rest for 15 minutes, stirring regularly, until very soft.

Step 7

Press the berries and juice through a strainer into a bowl using the bottom of a ladle. You should get about 1 3/8 cups of purée. Leave to cool completely, about 45 minutes.

Step 8

Whip the cream and yogurt together in a large bowl until soft peaks form.

Step 9

Taste the cold purée and add the 2 tbsp superfine sugar if needed. Pour it into the cream and stir just once or twice until very lightly combined and marbled.

Step 10

Spoon into individual wide glass dishes or one large bowl. Scatter with tiny mint leaves and the reserved whole blackberries. Sprinkle with a little extra sugar if liked. Serve with the hazelnut cookies.