1. First make the base. Place the crushed biscuits in a bowl; then add the melted butter and mix well. Press the mixture onto the base and sides of a pie dish with a one and one half-litre (five-cup) capacity. Refrigerate until ready to use.
2. In a saucepan, heat three cups of milk, reserving the remaining cup for later. Add the butter or margarine and condensed milk (or sugar), and keep stirring with a whisk. Just as it starts to come to a boil, remove from heat and set aside to cool slightly.
3. Meanwhile, combine the salt, custard powder, cornflour and beaten eggs with the reserved cup of milk. Whisk until smooth. Very slowly add the cooled milk mixture to the egg mixture (it mustn’t scramble the eggs), and keep whisking until well combined. Return this mixture to the saucepan in which the milk was warmed earlier. Heat and continue whisking until it thickens. (It’s important to whisk all the time as the mixture must be very smooth, with no lumps. If you find a few, strain the mixture through a fine sieve.) Finally, add the vanilla extract and stir well.
4. Pour the filling into the refrigerated pie crust. Sprinkle with cinnamon and allow to cool. Cover with plastic cling film and allow to set overnight.