Preheat your oven to 300 degrees and have a small sheet pan ready. Give your walnuts a rough chop, and put them on your baking sheet in a single layer. Bake for 12-15 minutes, or until they are fragrant and lightly toasted. Let the walnuts cool completely before adding to the cookie dough.

In a medium bowl, stir together the flour, old fashioned oats, salt, baking powder, and baking soda, and set the mixture aside. In the bowl of a stand mixer, using a metal paddle attachment, beat together the cold butter, sugar, and brown sugar. The mixture will become sandy and will still have small chunks of butter running throughout.

Beat in the eggs, and vanilla extract until combined. The mixture will be lumpy, but that's ok! Slowly beat in the flour mixture until well combined- this will take a bit of working! Then, stir in the toasted and cooled walnuts and chopped chocolate until well distributed.

Line a small baking sheet with parchment paper. Start forming your dough into balls by grabbing about a heaping 1/2 cup of dough and gently pressing it into a ball. You want to just gently press it together without rolling it or firmly forming the dough- it's okay if it's a little rough around the edges. The whole batch should make about 12-15 giant cookies!

Cover the tray with plastic wrap and freeze the cookies for at least 1 hour, and up to 3 months before baking. When you're ready to bake, preheat your oven to 375 degrees. Line a large baking sheet with parchment paper.

Place 6 dough balls onto the sheet evenly spaced. Bake the cookies for 16-18 minutes, or until the cookies are puffed and look dry on the top. They cookies will be very thick, and they may not be done in the middle yet, but they will continue to cook as they cool! Cool the cookies on the pan for 15 minutes, and then transfer to a cooling rack. Enjoy!