Defrost the frozen puff pastry tart shells.  Preheat the oven to 200 degrees C (approximately 400 F) and place the oven rack in the bottom third of the oven.  Over medium high heat, heat the water in a medium sized sauce pot until it starts to steam.  Then remove it from heat and stir in the icing sugar until it has completely dissolved.  In a large bowl, whisk the eggs with the evaporated milk until well combined and stir in the vanilla extract.  Pour the hot sugar water in a very thin stream to very slowly add it to the eggs and evaporated milk, whisking continuously.  Sift the egg mixture through a fine mesh sieve into a lipped measuring cup and set aside.  Place the defrosted puff pastry tart shells on a baking sheet.  Pour the egg mixture evenly into the tart shells up until the bottom of the ridges of the crust edge.  Bake the tarts in the oven until the edges are lightly golden, about 10 minutes.  Then lower the heat to 180 degrees C (approx. 350 F) and bake for 8 to 10 more minutes.  The custard should have puffed up into a dome.  Open the oven door, about 3 inches, and wait for the top of the custard to flatten out again.  This will only take a minute or two.  Then remove the tarts and transfer them to a wire rack to cool.  The custards are done if the middle of the custard is only slightly more jiggly than the edges of the custard and the edges of the pastry are lightly browned.  Enjoy warm (not hot) or at room temperature.  The custard will set further as it cools.