Preheat the oven to 350°F (175°C) and line two baking sheets with parchment paper.
In a large bowl, beat the softened butter, brown sugar and granulated sugar together until pale and creamy, about 2 minutes.
Mix in the egg and egg yolk one at a time, then stir in the vanilla until smooth.
In a medium bowl, whisk the flour, cornstarch, baking soda and salt.
Add the dry mixture to the butter mixture and stir just until no dry streaks remain.
Fold in the chocolate chips or chopped chocolate.
Cover the bowl and refrigerate the dough for at least 30 minutes.
Scoop the dough into balls of about 2 tablespoons each and place them 2 inches apart on the prepared sheets, about 8 per sheet.
Bake one sheet at a time for 10 to 12 minutes, until the edges are golden and the centers still look soft and slightly puffy.
Sprinkle with flaky salt if you like. Let the cookies rest on the sheet for 5 minutes, then move them to a wire rack.