In a small bowl, combine granulated sugar and hot water until the sugar dissolves. Set aside to cool.
Cut the puff pastry sheet into 9 squares. Press down the dough onto the lightly-greased tart molds. Cut the excess dough if necessary.
In a large bowl, beat together the sugar syrup, eggs, milk, and vanilla extract with a whisk until smooth. Strain the mixture twice to achieve a smooth egg mixture.
Pour the egg mixture into the tart molds
Bake for 30 minutes until the crust golden brown. Let it cool on the tart molds for 5 minutes. Remove the molds and transfer them to the wire rack. Best serve warm.
Notes
Once filled, immediately bake them in the oven. The filled egg tarts will get soggy when they are left too long outside the oven.The egg filling will puff out a lot during baking and then deflates when the egg tarts are taken out from the oven.They are best eaten within a day because the wet egg filling will make the crust soggy after some time.