Place custard powder, semolina and sugar in a medium sized saucepan. Gradually add enough milk to make a paste. Place over medium heat and whisk in remaining milk. Split the vanilla pod and scrape in the seeds, reserving the pod. Bring to the boil and cook until mixture thickens. If lumps appear just give the mixture a good whisk.
Preheat oven to 160°C.
Cut pastry sheets to the same size as dish; discard excess pastry. Brush 1 sheet of pastry with some of the butter; top with 7 more pastry sheets, brushing each with butter. Place in base of dish; spread custard into dish. Repeat with remaining pastry and butter; place on top of custard.
Place in an oven. Bake for 55 minutes, or until pastry is golden and custard is set.
Meanwhile, to make sugar syrup place sugar and water in a medium sized pan. Cook over low heat until sugar dissolves. Add the lemon zest and juice, plus the reserved vanilla pod. Simmer for 5 minutes or until mixture is thick.
Pour hot syrup over hot galaktoboureko; cool. Serve in slices, dusted with combined sifted icing sugar and cinnamon. Serve hot or cold. Alternatively you can serve the syrup separately.