For the crust:
For the custard:
1. Make the dough: In a food processor, combine the flour, sugar, and salt. Process few seconds until combined. Add cubed butter and pulse until crumbs are formed. And egg, vanilla extract and pulse until dough is formed.
2. Flatten into disk, wrap in plastic wrap and refrigerate for 30 minutes.
3. Make the filling: Combine hot water and white sugar in a small bowl, stir until the sugar is dissolved, set aside to cool to room temperature.
4. Beat the eggs with the milk in a large bowl. Pour in the cooled sugar water and mix until well combined.
5. Roll out the dough to a 1/2 cm thickness. Cut dough with a cookie cutter that is just a bit bigger than your tart tin in size. Line dough in the middle of tart tins, one by one. Lightly press the dough with your thumbs, starting from the bottom then up to the sides. Pierce the dough with a fork, refrigerate for 30 minutes.
6. Preheat oven to 400F (200C). place the tray in the lower rack.
7. Strain the filling through a sieve, and Pour into the shells until it is about 70-80% full.
8. Bake for 5-6 minutes, then reduce the heat to 350F (175C) and bake for 10 minutes more. until filling is just set.
9. Cool down for several minutes and then take the egg tarts out of the tins. Serve warm or cold.