Preheat your oven to 170°C and place 12 muffin cases in a muffin tray.
Put the castor sugar and eggs in a bowl. Beat for 2 minutes until pale. Add the lemon rind, vanilla extract and lemon juice and stir.
Beat in the melted butter and milk. Sift in the flour, baking powder and salt. Add the poppy seeds, then stir until just combined.
Spoon the muffin batter into the muffin cases. Bake for 15-20 minutes until an inserted skewer comes out clean.
Start preparing the syrup during the last 10 minutes of baking time. Combine all of the syrup ingredients in a small saucepan. Stir over a low heat until the sugar has dissolved. Increase the heat and allow to boil gently for 5 minutes.
Take the muffins out of the oven and spoon hot syrup over them. Allow to cool completely before serving.