Begin by melting your unsalted butter until it's just opaque and allow it to cool slightly. In a large mixing bowl, combine the melted butter with both brown sugar and granulated sugar. Using a whisk, mix them together for about two minutes until the mixture is smooth and well-blended.

Once your butter and sugars are combined, introduce the egg and egg yolk. Also, add in the vanilla extract. Whisk everything until the mixture is light and creamy.

In a separate bowl, whisk together your all-purpose flour, baking soda, corn starch, and salt. Gradually add this dry mixture to your wet ingredients. Mix just until everything is combined.

Fold in the semi-sweet chocolate chips until evenly distributed throughout the dough.

Scoop out about 1/4 cup of cookie dough for each cookie and roll them into smooth balls. Place them on a parchment-lined baking sheet and chill the dough in the refrigerator for one hour or in the freezer for 30 minutes before baking.

Preheat your oven to 325°F (160°C).

Once chilled, place your cookie dough balls on a baking sheet. Bake them for about 12 to 15 minutes until the edges are golden brown but the center still appears slightly undercooked.