Preheat oven to 180ºC. Brush 12 friand moulds with melted butter. In a jug, combine milk and poppy seeds and stand for 30 minutes.
Meanwhile, put lemons in a pan and cover with water. Bring to the boil, reduce heat and simmer, covered, for 20 minutes. Drain. Put lemons in a bowl of iced water for 5 minutes.
Drain lemons, cut into quarters and remove seeds. Put lemon, butter and caster sugar in a food processor, then process for 1 minute until almost smooth. Add egg and process for a further 30 seconds. Transfer to a bowl, add flour and milk mixture and stir until just combined. Spoon mixture into prepared moulds until two-thirds full and smooth the surface. Bake cakes for 45 minutes or until cooked. Leave cakes in moulds for 5 minutes before turning out onto a wire rack.
Line a tray with baking paper. Combine rind, extra sugar and water in a pan. Stir continuously over a low heat until sugar is dissolved. Bring to the boil, reduce heat and simmer for 5 minutes. Using a fork, remove rind from syrup and put on prepared tray.
To make lemon icing, put icing sugar and butter in a bowl, slowly add lemon juice and stir with a wooden spoon until mixture forms a thin paste. When cakes are cool, spoon a little icing over each cake and top with a few strands of rind. Stand for 10 minutes, to set.