In a medium bowl, stir together flour, cocoa powder, salt and baking soda. In a large mixing bowl, beat brown sugar, softened butter and vanilla until creamy. Add eggs and beat. Add dry ingredients and mix well. Stir in semi-sweet chocolate chips.

Cover and chill the cookie dough for at least 1 hour (dough will become stiff after chilling).

Drop mixture by rounded tablespoons on to ungreased cookie sheets. Bake in a 375 oven 10 minutes or until edges are firm. Cool one minute on cookie sheet, then remove to a wire rack to cool.