In an electric mixer fitted with the paddle attachment, cream the butter and sugars together on medium-high speed for 3 minutes. Scrape down the sides of the bowl.
Add the egg and vanilla and continue beating on medium-high speed for 4-5 minutes, until the mixture is very light and fluffy. Make sure to pause half way to scrape down the sides of the bowl.
In the meantime, combine the flour, milk powder, salt, baking soda, and baking powder together in a medium bowl. Add cinnamon and nutmeg as well if using.
Add the dry ingredients into your mixer and beat on low speed until flour is evenly distributed. It is okay if there are still white spots.
Add the chocolate chips and walnuts and continue mixing on low speed until they are evenly distributed and the flour is now fully combined. If possible, chill the dough for at least an hour before baking.
When you're ready to bake, preheat your oven to 375°F and line two cookie sheets with parchment paper. Scoop 2 tbsp-sized balls of dough and space them at least 2 inches apart. Bake for 12-13 minutes, or until edges are golden brown. Let the cookies rest for 10 minutes on the sheets before transferring them to wire racks to cool completely.