A full teaspoon of salt in the cookie dough recipe makes all the difference. These are the best chocolate cookies!
Preheat oven to 350 degrees. Add the butter, sugar, brown sugar, salt, baking powder and baking soda to the bowl of a stand mixer. Mix on low until combined and then beat on medium speed for about 1 minute. Beat in egg and vanilla until smooth. Stop mixer and add the flour, walnuts and bittersweet chocolate chip. Mix on low speed until a cohesive cookie dough forms, about 1 minute.
Drop dough by a cookie scoop (1 1/2 tablespoons each) onto cookies sheets lined with parchment paper or silicone baking mats. Bake for 10 minutes. I rotate the pans front to back and top rack to bottom rack halfway through baking time.
The cookies should look a bit underdone when you take them out of the oven. They will set up as they cool.
Allow the cookies to cool on the baking sheets for 10 minutes. Remove them to a cooling rack and allow to cool to room temperature.
Cookies are best eaten within a day or two. Place completely cooled cookies in a freezer bag and freeze for up to one month if desired.