Add the cold butter, granulated sugar, and brown sugar to a large bowl or stand mixer. Using the paddle attachment or beaters, mix on low speed until the butter and sugar are incorporated but not creamed, for about 1 minute.
½ cup unsalted butter, ½ cup granulated sugar, ½ cup brown sugar
Add the flour, baking powder, and salt to the mixture. Mix on low speed until it resembles small crumbs, about 1-2 minutes.
1½ cups all-purpose flour, ½ teaspoon baking powder, ½ teaspoon kosher salt
Mix in the chocolate chips and chopped walnuts (if using) on low speed until they are evenly distributed throughout the dough crumbs.
1 cup semi-sweet chocolate chips, ½ cup chopped walnuts
Add the egg and mix on low just until the dough comes together, about 1 minute.
1 large egg
Cover the bowl tightly with plastic wrap and place the dough in the fridge to chill overnight (minimum of 8 hours, or up to 72 hours for even more flavor).
When ready to bake, preheat your oven to 350°F. Line a baking sheet with parchment paper.
Divide the chilled dough into uneven pieces.
Gently press the dough pieces together, shaping them into 6 rough dough balls (about 130 grams per ball). Place them on the prepared baking sheet, leaving ample space between each for spreading during baking.
Bake in the preheated oven for about 12-15 minutes, or until the edges are lightly golden but the centers are still soft. If desired, sprinkle a pinch of flaky sea salt on top of each cookie.
Flaky sea salt
Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.