Make the pastry. In the bowl of your stand mixer, add the margarine, butter blend, flour, and salt.
Using the beater attachment, mix at the lowest speed, until the mixture resembles coarse breadcrumbs.
While the mixer is mixing, in a separate container, add the iced water, eggs, and sweetened condensed milk. Mix to combine.
Continuing at the lowest speed, pour the liquid mixture into the mixing bowl. Mix until just combined and a dough is formed. Be careful not to overmix.
Divide the dough into 18-gram pieces. You should end up with 48 pieces. Roll each piece of dough into a ball.
Press the dough pieces into the tart moulds.
Arrange the moulds in a baking tray [See Note 2]. Set aside while you make the filling. Optionally, let rest for 1 hour.
Next, make the filling. In a large mixing bowl, add the milk or water, sweetened condensed milk, sugar, egg yolks, vanilla extract, cornstarch, and salt. Beat gently, until throughly combined the sugar is dissolved. Optionally, let rest for 1 hour, before proceeding with the next step.
Preheat the oven to 160 degrees C, fan off. Place the rack on the bottom rung of the oven [See Note 1].
Using a fine mesh strainer, strain the mixture.
Ladle the filling into the tart shells, about a tablespoon each. You should have just enough filling, with no leftovers.
Bake in the preheated oven, until slightly caramelized around the edges of the filling, about 1 hour.