In a large bowl, cream together the butter, brown sugar and white sugar until light and fluffy. Add eggs one at a time. Add vanilla.
In another bowl, combine the flour, cocoa, salt and baking soda and gradually add to the creamed mixture. Fold in chocolate chips. Mixture will be thick! Roll dough into tablespoon sized balls. Place on baking sheet and press down slightly.
Bake for 10-12 minutes until puffy but still soft. Allow cookies to cool on baking sheet for 5 minutes before removing to a wire rack to cool completely.