In a large bowl, using a handheld mixer or a stand mixer fitted with a paddle attachment, cream the butter with the sugar, egg yolk, salt, and vanilla extract until fluffy. Add the milk and baking powder (if using), and mix well.
Slowly add in the all-purpose flour and cornstarch, beating until the dough becomes soft and no longer sticks to your hands.
Preheat the oven to 350°F (176°C). Line two baking sheets with parchment paper or a silicone baking mat. Cover the dough with plastic wrap and chill in the refrigerator for 30 minutes.
Transfer the cookie dough to a cookie press, following the manufacturer's instructions. Choose a festive disk, such as a Christmas tree. Press out the cookies onto the prepared baking sheet, spacing them about 1 inch apart. Sprinkle the tops of the cookies with festive sprinkles.
Place the cookies on the prepared baking sheets and bake for 15 minutes, or until they turn very light brown and the insides are cooked through. Allow the cookies to cool on a rack. Store them in an airtight container.
Notes
Please use the metric measurements for the most precise results.