In a small bowl combine flour, cornstarch and baking soda. Whisk gently. Set aside.
Melt butter slightly in microwave. It should be runny but not completely liquefied. In a mixing bowl fitted with flat paddle cream butter with brown sugar and granulated sugar. Beat on medium speed for until well mixed. Stir in egg, egg yolk and vanilla.
Add flour mixture and beat for 1-2 minutes until thoroughly combined. Mix in chocolate chunks.
Refrigerate dough for at least 2 hours or up to 2 days.
Use your fingers to scoop dough into a rough 1 1/2 inch rounds . Don't smash it into a ball. Place on non-stick baking sheet or parchment lined cookie sheet pan.
Bake for 9-11 minutes or until edges have just begun to brown. Centers will be glossy and look slightly under cooked. Sprinkle with salt. Let them cool on pan for about 5 minutes. Remove to rack to cool completely.
Will keep fresh for up to 1 week in covered container.
*Plan ahead. This dough tastes best chilled for at least 2 hours prior to baking. Not mandatory though.