Preheat oven 150ºC . In med bowl combine flour, soda, salt, and cocoa. Mix well with wire whisk and set aside. In lg bowl blend sugars with mixer at med speed. Add butter and beat to form a grainy paste. Scrape sides then add eggs and vanilla. Beat at med speed until light and fluffy. Add flour mixture and chocolate chips. Blend at low speed until just combined. Do not overmix. Drop dough by rounded Ts onto ungreased cookie sheets 4 cm apart. Bake 21-23 mins. Immediately transfer to a cool surface.