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, sweetened condensed milk, and milk paste if using. 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.
Arrange the moulds in a baking tray [See Note 1]. Arrange the dough balls in the center of each mould.
Use your fingers to press it down into the base, pushing it toward the edges. Let rest, in the fridge is possible, 1 hour.
After resting, using a flour-dusted tart tamper, push down into the center and work your way out toward the sides. Use the smaller end of the tamper to press the dough firmly into the corners and up the ridges. Let rest again, in the fridge if possible, another 30 minutes.
Next, make the filling. In a large mixing bowl, add the milk or water, sweetened condensed milk, sugar, egg yolks, vanilla extract, milk paste if using, cornstarch, and salt. Beat gently, until throughly combined the sugar is dissolved. Mixing gently prevents any bubbles from forming.
Let rest before proceeding with the next step, 30 to 60 minutes. Resting the mixture allows the sugars to fully dissolve and give the filling a caramelized finish along the edges when baked.
Using a fine mesh strainer, strain the mixture. Repeat this step twice for best results.
Preheat the oven to 170 degrees C, fan off. Place the rack on the bottom rung of the oven.
Using a fork, prick the bottom of each pie shell. This will prevent the crust from puffing up when baking.
Blind bake the shells in the oven, until the edges are set and the crust is pale golden, 15 minutes. Remove from the oven, and reduce the temperature to 160 degrees C.
Skim off any foam from the filling mixture. Ladle the filling into the tart shells, about a tablespoon each. You should have just enough filling, with no leftovers.
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 filling is set and slightly caramelized around the edges, about 45 minutes.