Make the shortcrust pastry dough. Combine all-purpose flour with powdered sugar, and salt. Add butter and blend in the ingredients with a pastry cutter or food processor until the mixture resembles coarse sand. Add egg yolk and water, mixing just until the dough comes together. Shape the dough into a disc and place between two sheets of parchment paper. Roll out the shortcrust pastry until it's about ⅛ inch thick. Place on a baking sheet and chill in the fridge for 1 hour or until firm.
Meanwhile, make the filling. Combine sugar with water and bring to a boil. Turn off the heat and add 2 bags instant milk tea powder and loose black tea leaves. Let sit for 10 minutes. Cool completely.
Whisk together 4 eggs with evaporated milk and vanilla extract. Add cooled milk tea mixture, whisking until well combined. Strain custard filling and set aside.
Remove dough from the fridge and peel off top parchment paper. Cut out circles with a 3-inch biscuit cutter and line 2 ½-inch mini tart shells to create the crust. Chill for 30 minutes or until firm.
Preheat oven to 350 degrees F.
Remove shells from fridge and fill with milk tea custard until it's about 80% full. Bake the tarts for 10-12 minutes or until the edges of the custard have set. The center will still be a little jiggly.
Let milk tea custards slightly cool. Serve warm.