Beat room temperature butter with an electric mixer for 2 minutes
Sift in powdered sugar and continue mixing for another 2 minutes
Add the egg and whisk until smooth
Sift in 1 cup of flour and whisk, then sift in the remaining cup and stir with chopsticks until crumbly
Knead the mixture by hand into a dough ball for about 3 minutes
Place dough in a Ziploc bag and refrigerate for 30 minutes
Roll the chilled dough out on a floured surface to ⅕-inch thickness
Use a mold or cutter to shape rounds and press each dough circle into a tart mold
Refrigerate tart shells until ready to fill
Dissolve sugar in hot water and let cool for 30 minutes
Whisk together eggs, evaporated milk, and vanilla extract
Add the cooled sugar water and whisk to combine
Strain the mixture twice to ensure a smooth texture
Place tart molds on a baking tray and pour custard filling into each tart shell about 80% full
Bake at 400°F (204°C) for 10 minutes on the lower third rack
Reduce heat to 325°F (163°C) and bake for another 10 minutes until the filling is just set
Turn off the oven, leave the door slightly open, and let rest inside for 5 minutes before removing
