Draw three 20.5 (8 inch) circles on non-stick baking parchment. Place the parchment circles on baking sheets.

Grind the walnuts finely in a blender or food processor.

Cream the butter and sugar together in a mixing bowl until pale and fluffy, then beat in the walnuts and flour. Divide the dough into 3 shortbread portions.

Put a portion of shortbread dough in the centre of each parchment circle and press out with the heel of your hand until the dough is the same size as the circle.

Cut one of the circles into eight triangles with a sharp knife and ease them slightly apart. Refrigerate the circles and triangles for 30 minutes. Bake at 190°C (375°F) mark 5 forl5-20 minutes, swapping over the sheets to ensure the pastries brown evenly. Leave to cool and harden for 10 minutes on the paper, then transfer to wire racks to cool completely.

Meanwhile, reserve one-third of the raspberries for decoration. Put the rest in a bowl with the icing sugar and liqueur, if using. Crush the raspberries with a fork, then leave them to macerate while the pastry rounds are cooling.

Assemble the shortbread just before serving, to ensure that the pastry remains crisp. Whip the cream until thick, then fold in the crushed raspberries and juice. Stand one round of pastry on a flat serving plate and spread with half of the cream mixture. Top with the remaining round of pastry, then the remaining cream mixture.

Arrange the triangles of pastry on top of the cream, wedging them in at an angle. Scatter the reserved whole raspberries in between. Serve the shortbread as soon as possible.