In a bowl, mix together the condensed milk and cake flour until no lumps remain. Set aside.
Heat up the milk and water in a saucepan and bring to a simmer. Pour the hot milk into the condensed milk flour mixture and mix well.
Pour the mixture back into the saucepan and add in the butter. Stir constantly over low heat until it starts to thicken.
Pour into a bowl and cover tightly with plastic wrap - make sure to press down the plastic wrap onto the custard to prevent the top from drying out. Refrigerate.
Custard Lava French Toast
Spread the cooled custard all over one piece of bread, while putting more in the centre. Place the other slice on top, press down, and seal all the sides together tightly (to prevent the custard from falling out when cooking).
In a wide dish, whisk together the egg and milk.
Soak the “sandwich” in the mixture for a minute, flip, and do the same for the other side and also the 4 sides.
Melt the butter in a pan over low heat. Fry the bread for about 2 minutes on each side (don’t forget about the 4 sides too).
Then turn off the heat, and cover for a few minutes (to ensure that the filling will flow out like lava). Plate, slice, and enjoy!