Whisk the flour, baking soda, cornstarch, and
salt together in a large bowl. Set aside.
In a medium bowl, whisk the melted butter,
brown sugar, and granulated sugar together
until no brown sugar lumps remain. Whisk in the
egg, then the egg yolk. Finally, whisk in the
vanilla extract. Pour the wet ingredients into
the dry ingredients and mix together with a
large spoon or rubber spatula. The dough will
be very soft, yet thick. Fold in the chocolate
chunks. They may not stick to the dough because
of the melted butter, but do your best to
combine them. Cover the dough and chill in the
refrigerator for 2-3 hours or up to 3-4 days.
Chilling is mandatory.