Preheat oven to 200°C. Cut each pastry sheet into a 25cm square, place each square on a baking tray lined with baking paper and prick all over with a fork. Top each square with another sheet of baking paper and another heavy oven tray. Bake, swapping and turning trays halfway through cooking so pastry cooks evenly, until dark golden and puffed (20 minutes). Cool pastries on wire racks.
Meanwhile, line a 20cm-square cake tin with baking paper, extending paper beyond the sides to assist with removal. Place the base of the cake tin on a cooled pastry square, then using a serrated knife, trim to the same size. Place one pastry square in tin.
Bring milk, cream and lemon verbena just to a simmer in a saucepan over medium heat. Remove from heat, cover and set aside to infuse (30 minutes). Meanwhile, whisk sugar, cornflour and yolks in a bowl to combine. Discard lemon verbena, then pour milk mixture into egg mixture, whisking continuously. Return mixture to pan with butter and whisk until thick and flour is cooked out. (4 minutes). Remove from heat and pour onto pastry in cake tin. Press remaining pastry square on top and refrigerate until set (3 hours).
For lemon icing, stir ingredients in a bowl until smooth. Lift slice from tin and spread with icing. Return to the fridge to set (30 minutes). Cut slice into squares and drizzle with passionfruit pulp.