Add 35g sifted powdered sugar and ⅛ tsp of salt to 100g softened butter. Mix with hand mixer on medium speed for 2 minutes until the butter turns pale yellow and fluffy.
Add ⅓ of the egg (10g) to the butter-sugar mixture, mix with hand mixer on medium speed for about 30 seconds, until all the egg liquid has been absorbed into the butter-sugar mixture. Add another ⅓ of the egg to the butter-sugar mixture, mix with hand mixer ... Repeat until all the egg has been mixed into the butter-sugar mixture.
Add 1 tsp of vanilla extract to the mixture, roughly mix with hand mixer on medium speed for about 5 seconds.
Sift in 160g cake flour and 15g milk powder to the mixture, roughly mix with a spatula until crumbly.
Use your hands to mix the dough until there is no dry flour. Transfer the dough to a plastic food wrap.
Place the dough in the center of the food wrap, fold the food wrap in half. You should get a rectangle half the size of your original food wrap. Make ½-inch-wide flaps by folding down the three open edges of the rectangle.
Use a rolling pin to roll the dough into a rectangle. Lay the rectangle dough flat on a plate, keep it in the freezer for about 30 min (or in the fridge overnight) until the dough turns hard.
Lay a layer of food wrap on the work surface, place the hardened egg tart dough on the food wrap, cover with another layer of food wrap over the top. Use a rolling pin to roll the dough into a 4 mm thick sheet. (If the dough feels to hard to be rolled, you can rest it at room temperature for about 3 minutes. But don't rest it for too long, otherwise it will become soft and sticky.)
Use a cookie cutter to cut out a disk from the dough sheet. Place the dough disk into an egg tart tin. Gently press the sides and the bottom of the egg tart shell to the tin, so that there is no air between the shell and the tin. Repeat until you use the whole sheet of the dough. (Always keep the rest of the dough sheet in the fridge while you are shaping an egg tart shell.)