1. Heat the condensed milk, milk, vanilla and cinnamon together to just below boiling point. Beat the eggs, cornflour and custard powder together with two tablespoons of water until smooth, then gradually beat in the hot milk until smooth. Pour back into the saucepan and heat until the mixture has thickened (about 5 minutes).
2. Arrange 12 of the biscuits on the bottom of a rectangular pie dish and spoon half of the custard on top. Smooth the surface and arrange the other 12 biscuits on top of the custard layer. Spoon the remaining custard on top and smooth again. Allow to cool completely and garnish with extra cinnamon and some edible glitter if desired.