Preparation

• Sift the cornstarch into a bowl to aerate it and remove any lumps. Measure out all remaining ingredients and set them nearby. Place your oven rack in the middle position and preheat 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 ingredient mixture, stirring gently until a smooth dough forms. Avoid overworking the dough.
• Roll the dough into walnut-sized balls and place them on parchment-lined baking sheets. Press each ball gently with your thumb and create a crosshatch pattern using a fork. Refrigerate for 30 minutes.
• Bake the chilled cookies in the preheated oven for 12-15 minutes until edges are light golden but centers remain pale and soft.
• Let the cookies cool on the baking sheet for 20 minutes, then transfer to a wire rack to cool completely.