Make the filling: in a medium to large sized saucepan, add the milk, vanilla pod seeds, semolina, caster sugar, grated lemon and the cinnamon stick. Whisk well to combine (you may find the cinnamon stick gets in the way) and cook, constantly stirring, over medium heat. When it reaches a gentle boil, reduce the heat to low and simmer (still stirring) until thickened. Remove from heat and discard the cinnamon stick. If using the 1/3 cup butter option, add it in now and stir through. Still stirring, add in the aquafaba and stir until well combined. The custard should be cooled (a little warm is okay) before pouring it in to the prepared filo. Set your custard aside.