Preheat your oven to 350°F (175°C). Spread the walnuts evenly on a baking sheet and toast them for about 8-10 minutes, until fragrant and lightly golden.

In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar together on medium speed for 2-3 minutes until light and fluffy.

Add the egg, egg yolk, and vanilla extract to the creamed mixture and beat until well combined.

Whisk together the flour, cornstarch, baking soda, and salt in a separate bowl. Gradually add to the wet mixture, mixing on low speed until a soft dough forms.

Gently fold in the chocolate chips and toasted walnuts until evenly distributed.

Scoop the dough into balls, place them on a baking sheet, and refrigerate for at least 30 minutes.

Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

Place six dough balls on each baking sheet and bake for 11-13 minutes until lightly golden at the edges.

Allow the cookies to cool on the baking sheet for about 5 minutes before transferring to a wire rack.