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. This texture is crucial—you want distinct butter pockets throughout the dough, not a smooth paste. Then, stir in the sweetened condensed milk and vanilla extract until the dough just comes together. I like to use a gentle hand here; overmixing develops gluten and makes the cookies tough instead of delicate and crumbly.

Working quickly while the dough is still cool, roll the dough into 1-inch balls using your palms and place them about 1 inch apart on your prepared baking sheet. The dough will be soft and slightly sticky, which is normal. Bake at 350°F for 12-15 minutes, until the bottoms are light golden brown but the tops remain pale—don't overbake, as these cookies continue to firm up as they cool.

Remove the cookies from the oven and let them cool on the baking sheet for 5 minutes. They should still be warm and slightly soft at this point. Sift the powdered sugar into a shallow bowl. Working with just a few cookies at a time, gently roll the warm cookies in the powdered sugar, coating all sides thoroughly. This first coating while warm helps it adhere and creates a light, delicate layer. Set the coated cookies on a wire rack to cool completely.

Once the cookies are completely cool (about 1-2 hours), sift the remaining powdered sugar and give each cookie a second generous coating, rolling them again until they're completely covered in a thick, snowy layer. This double-coating technique ensures those beautiful frosted snowball cookies with a luxurious powdered sugar exterior. I find the second coating sticks best when the cookies are fully cooled because there's less moisture on the surface.