Preparation

• Preheat your oven to 350°F and line a baking sheet with parchment paper. Cube your cold butter into small pieces—keeping it cold is essential for creating a tender, crumbly cookie texture. Sift together the all-purpose flour, salt, and cornstarch (if using) in a small bowl to remove any lumps and ensure even distribution.
• In a medium bowl, combine the cubed cold butter with the dry ingredient mixture from Step 1. Using a pastry cutter, two knives, or your fingertips, work the butter into the flour until the mixture resembles coarse breadcrumbs or wet sand. Stir in the sweetened condensed milk and vanilla extract until the dough just comes together.
• Roll the dough into 1-inch balls using your palms and place them about 1 inch apart on your prepared baking sheet. Bake at 350°F for 12-15 minutes, until the bottoms are light golden brown but the tops remain pale.
• Remove the cookies from the oven and let them cool on the baking sheet for 5 minutes. Sift the powdered sugar into a shallow bowl and gently roll the warm cookies in it, coating all sides thoroughly.
• Once the cookies are completely cool (about 1-2 hours), sift the remaining powdered sugar and give each cookie a second generous coating, until they're completely covered.