Preheat your oven to 350°F and line a baking sheet with parchment paper or a silicone mat. This ensures your cookies won't stick and allows for even browning on the bottom. Having everything ready before mixing means you can work quickly with the dough while it's at the perfect consistency.

In a medium bowl, combine the softened butter and sweetened condensed milk, mixing until well combined and slightly fluffy, about 1-2 minutes. The butter should be at room temperature to blend smoothly with the condensed milk. This creates a creamy base that will help bind the dry ingredients.

Add the sifted cornstarch, vanilla extract, and fine sea salt to the butter mixture from Step 2. Stir until a thick, cohesive dough forms. If the dough feels too soft or sticky, add the additional cornstarch a tablespoon at a time until you reach a consistency that holds together but isn't crumbly. I find that the dough should feel similar to play dough—firm enough to shape but still slightly moist.

Using your hands or a small cookie scoop, roll the dough into balls about the size of walnuts and place them on the prepared baking sheet, spacing them about 2 inches apart. Gently press each ball into a thick disk roughly 1/4 inch thick using the bottom of a glass or the heel of your hand. I like to lightly dampen my fingers before pressing to prevent sticking and create an even, slightly rustic appearance.

Bake for 13-15 minutes until the edges turn light golden brown while the centers remain pale. The cookies will firm up as they cool, so don't overbake—they should still feel slightly soft when you remove them from the oven. Keep an eye on them during the last few minutes, as they can brown quickly.

Let the cookies cool on the baking sheet for 5 minutes to set slightly, then transfer them to a wire rack to cool completely. Once cooled, they'll have a delicate, shortbread-like texture. Store in an airtight container for up to 5 days.