In a mixing bowl, combine the room temperature unsalted butter, sweet condensed milk, and vanilla extract. Blend on medium speed until smooth, allowing some small lumps of butter to remain for texture. This will take about 2-3 minutes.

Gradually sift in the cornstarch, mixing with a spatula until the dough becomes too thick to stir with a spoon. Switch to using your hands, gently kneading until a soft and smooth dough forms.

Divide the dough in half, roll each half into a log about the thickness of your index finger, and slice into ¼" thick pieces. Roll each piece into a ball and place them on a parchment-lined baking sheet.

Using a fork, gently flatten each ball to create a slight indentation and refrigerate the cookies for about 30 minutes.

Preheat your oven to 350°F (175°C). Ensure your baking sheets are ready for the cookies.

Remove the chilled cookies from the refrigerator and bake for about 10-12 minutes, until the bottoms are lightly golden.

Let the cookies cool on the baking sheet for about 5 minutes, then transfer them to a wire rack to cool completely.