In a large bowl, add butter and salt. Using a hand or stand mixer on medium speed, cream for 3 minutes until light and fluffy.
Add in milk powder, powdered sugar, and vanilla, then mix again on medium speed until well combined.
Add flour and mix on low speed until the mixture starts to clump together into a dough. Stop mixing as soon as it comes together — don’t overmix (see video for visual demonstration). Bring the dough together to form a ball, without overworking or kneading the dough.
Place the ball of dough onto a piece of parchment paper and roll it into a circle or rectangle (depending on the final cookie shape you want). The rolled dough should be about ¼ inch thick. Sprinkle flour on the dough as necessary to prevent it sticking to the rolling pin.
Transfer the rolled dough (by dragging the parchment) onto a baking tray and chill for 30 minutes. Meanwhile, preheat your oven to 180°C (350°F) conventional (see note 1 if using a convection oven with a fan).
Once chilled, cut the dough into your desired shapes. Re-roll any scraps to make more cookies if needed. If the dough softens too much, chill it again briefly.
Prick the tops of the cookies with a fork for that classic shortbread look if desired, then place them on a lined baking tray, leaving a small gap between each one.
Bake for 12–13 minutes, or until the edges are lightly golden. For a slightly crispier texture (which I prefer), bake for an extra minute or two, or until the edges are a deeper golden brown color.
Turn off the oven and leave the door slightly open, letting the cookies cool inside for 15 minutes (see note 2). Remove tray from oven and transfer the cookies to a wire rack to cool completely before serving.