Prepare a baking sheet by lining it with parchment paper or a silicone baking mat. Set aside. Heat the oven to 325°F.
Add 1/2 cup softened salted butter and 1/2 cup sweetened condensed milk. Mix on low speed with a hand mixture (or by hand) until the butter and milk are creamed together.
Fold in 2 cups cornstarch with a spatula until the cornstarch is mostly mixed into the sweetened condensed milk mixture, then mix on low speed with a hand mixer until the dough is well-mixed and resembles play-doh.
Use a spoon (or a cookie scoop) to scoop the dough in approximately 1-2 Tablespoon portions and roll the portions into balls between the palms of your hands. Place the dough balls onto the prepared cookie sheet, spacing them about 1 1/2"(about 4cm) apart.
Bake for about 15 minutes, until the dough is set (note that the cookies won't really brown, except on the bottoms - the tops will remain a creamy off-white color). Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely. Store any leftovers in an airtight container at room temperature for up to 2 days.
Notes
If you use unsalted butter, add a pinch or up to 1/4 teaspoon of salt to with the butter in step 2.