Line a baking sheet with parchment paper and set aside.

In an electric mixer, beat cold butter on medium speed until it forms a cohesive mass (about 1 minute).

Add brown sugar and granulated sugar, beating for 1-2 minutes until incorporated.

Gradually add lightly beaten eggs and vanilla extract; continue beating until mixed, scraping the bowl's sides.

Reduce speed to medium-low and gradually add flour mixture until only a little flour remains.

Stir in dark chocolate chips (and chopped walnuts, if using) with a rubber spatula.

Divide the dough into 12 even pieces and roughly shape into balls (don't smooth).

Place evenly spaced on the prepared baking sheet.

Refrigerate the cookies for 30 minutes.

Preheat oven to 375 degrees F (190 degrees C).

Bake for 18-24 minutes, or until lightly golden brown on top.

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

Store leftovers in an airtight container at room temperature for up to 4 days.