Heat oven to 350 degrees F (177 degrees C).
Grab a large bowl then use a hand mixer to beat the butter until whipped and creamy. This takes about 30 seconds. Now, with the mixer on, gradually add the sugar and beat into the butter until combined. Add the egg and vanilla and beat until blended.
Sift the flour, cocoa powder, baking soda and the salt into a a separate medium bowl, and then set aside.
Grab a spoon and stir about 1/3 of the sifted flour mixture into the butter mixture. Once incorporated, add another 1/3 and stir until combined. Then, add the final third and stir until combined.
Add walnuts and chocolate chips then stir until incorporated. Then, drop cookie mixture by rounded tablespoonfuls onto baking sheet(s). Then, gently press the cookie dough down towards the baking sheet.
Bake cookies for 8 to 10 minutes or until edges are dry and tops are slightly cracked. Cool on baking sheets.
Store cookies in an airtight container for up to 3 days at room temperature or up to one week, if kept in the refrigerator.