Beat the softened unsalted butter, packed light brown sugar, and granulated sugar with a hand mixer on medium speed for 2 minutes, until smooth and slightly fluffy. Scrape the bowl.

Beat in the egg, egg yolk, and ground vanilla bean until fully combined, about 30 seconds. Scrape the sides and bottom of the bowl.

In a second bowl, whisk the all-purpose flour, cornstarch, baking soda, and fine salt.

Mix the dry ingredients into the butter mixture on low speed just until a few flour streaks remain. Fold in the semisweet chocolate chips with a spatula until the dough is uniform.

Scoop 24 portions of about 1 1/2 tablespoons each onto a parchment-lined tray. Cover and refrigerate for 30 minutes. Meanwhile, heat the oven to 350°F with a rack in the center.

Arrange 12 chilled dough balls at least 2 inches apart on a parchment-lined baking sheet. Bake for 11 to 13 minutes, until the edges are light golden and the centers look set but soft. Keep the remaining dough chilled and bake it on a cool second sheet.

Let each batch stand on its baking sheet for 5 minutes, then transfer to a wire rack for at least 10 minutes. The centers finish setting as the cookies cool.