Sift the cornstarch into a bowl to aerate it and remove any lumps. Measure out all remaining ingredients and set them nearby. Preheat the oven to 350°F.

In a medium bowl, combine the softened butter, sweetened condensed milk, vanilla extract, egg yolk, and salt. Mix until pale and smooth—about 1-2 minutes.

Gradually fold the sifted cornstarch into the wet mixture until a smooth dough forms. Avoid overworking the dough.

Roll the dough into walnut-sized balls and place them on parchment-lined baking sheets. Flatten gently with your thumb and create a crosshatch pattern with a fork. Refrigerate for 30 minutes.

Bake for 12-15 minutes, until the edges are light golden but the centers are still pale. Check at 12 minutes to avoid overbaking.

Let the cookies cool on the baking sheet for 20 minutes before transferring to a wire rack to cool completely.