Preheat oven to 350 degrees. Grease 1-2 large cookie sheets.
In a small bowl, combine flour, cocoa, baking soda and salt. Set aside.
Using a hand or stand mixer, beat together the butter, brown sugar, sugar and vanilla.
Beat in the eggs one at a time, until fully combined.
Gradually beat in the flour mixture from step 2. Stir in the chocolate chips, oats and walnuts, if you're using them.
Drop by rounded tablespoons onto greased cookie sheets. Cook for 10-12 minutes. Allow to cool for a few minutes before transferring to a cooling rack.