Place butter at room temperature until softened. Cream the butter and sugar with an electric mixer over medium speed until the mixture is smooth, fluffy and light in color.
Add in whisked egg, half at a time, beat over low speed. Add vanilla extract, mix well.
Sift in flour in two batches, scraping down the sides of the bowl between additions with a spatula, and make sure all ingredients combine well. Knead into dough. (see picture)
Roll out the dough to a 1/2 cm thickness. Cut dough with a cookie cutter that is just a bit smaller 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. While pressing the dough, turn the tart tin clockwise/anti-clockwise in order to make an even tart shell. Trim away any excess dough.
Add sugar into hot water, mix until completely dissolved.
Whisk egg with evaporated milk. Pour in sugar water. Mix well.
Sift egg mixture to get rid of any foam into a tea pot. Carefully pour egg mixture into each tart shell.
Method (baking tarts):
Preheat oven to 200C. Position rack in lower third of oven. Bake tarts for 10 to 15 minutes until the edges are lightly brown.
Lower the heat to 180C. Keep an eye on them. Once you see the custard being puffed up a bit, pull the oven door open about 2 to 3 inches. Bake for another 10 to 15 minutes until the custard is cooked through. Just insert a toothpick into the custard. If it stands on its own, it’s done.
Notes:
Placing tarts at the lower rack in oven could cook crusts easily before the egg custard being heated up too quickly. (My oven will automatically turn on the heater at the bottom when it turns to “bake mode”. It’s better to check the menu of your own oven and know how your oven is designed.)
At the very last stage, pull the oven door open a few inches. This method is to avoid custard from being puffed up too high. The custard would collapse once they are cooled down otherwise.