Prep time 35 mins, cook 50 mins (plus cooling)
Bring milk, sugar, cinnamon, vanilla bean and seeds to the boil in a saucepan over low heat, then whisk in semolina and a pinch of fine salt, whisking continuously until thick (4-5 minutes). Whisk in 20gm butter, remove from heat, cover closely with plastic wrap and cool to room temperature. When cool, remove vanilla bean, stir through eggs and set aside.
Meanwhile, place remaining butter in a saucepan over low heat and skim occasionally until milk solids separate and butter is clarified (10-12 minutes). Strain through a fine sieve lined with muslin (discard solids) and keep warm.
Preheat oven to 165C. Brush a 5cm-deep, 25cm x 35cm baking tray with clarified butter, trim filo to size, then line base of tray with seven sheets of filo, brushing sheets with clarified butter as you layer them. Pour in semolina mixture, smooth top, then top with remaining filo sheets, brushing each with butter. Brush top with butter and bake until pastry is golden, custard is set and an inserted knife withdraws clean (40-45 minutes).
Meanwhile, for sugar syrup, combine sugar, lemon rind, lemon juice and 200ml water in a small saucepan over medium heat, bring to the boil, then simmer until slightly reduced (3-5 minutes). Strain hot syrup over galaktoboureko as soon as it comes out of the oven, then set aside to cool to room temperature. Serve dusted with icing sugar and cinnamon. Galaktoboureko will keep stored in an airtight container for 2 days.