Line 2 cookies sheets with parchment paper and set aside.
To a medium bow, add flour, baking soda, and cornstarch. Sift once and add the kosher salt. Set aside.
In a bowl of an electric mixer fitted with a paddle attachment, add the somewhat cooled melted butter, along with the sugar and dulce de leche. Mix on medium speed until combined and lightened, about a minute and a half. Add the eggs one at a time, beating well after each addition. Reduce the mixer speed to low and add in the vanilla extract. Stop and scrape the mixer.
Turn the mixer on to low speed and add the flour mixture all at once. Mix until the flour is mostly moistened. If there are dry bits of flour, it's fine.
Fold in the chocolate chips, making sure to scrape the bottom of the bowl when you fold to incorporate everything. Cover the bowl with plastic wrap and chill in the fridge for about 30 minutes to an hour.
Preheat the oven to 350 degrees Fahrenheit and remove dough from the refrigerator. Using a mini cookie scoop, scoop spoonfuls of dough, roll them into bowls and place on the baking sheet about 2 inches apart.
Bake for 7-8 minutes (I prefer 7 for that underbaked goodness situation), rotating the cookie sheets halfway through to ensure even baking. The centers of these cookies will look underbaked and that's what you're looking for.
Remove cookies from the oven and allow to cool for 3-4 minutes, then transfer to a wire rack to cool completely.
Recipe adapted from Host The Toast