In a medium bowl, combine flour, cocoa powder, baking soda and salt. Set aside.
In a large mixing bowl, beat margarine, granulated sugar and brown sugar with an electric mixer on medium speed until mixture is combined, scraping sides of bowl occasionally. Beat in eggs and vanilla until combined. Beat in as much of the flour mixture as you can with the mixer on low speed. Stir in any remaining flour mixture with a wooden spoon until combined. Stir in chocolate pieces. If necessary, cover and chill dough until easy to handle.
Drop dough by rounded tablespoons or by using an ice cream scoop 4 inches apart onto ungreased cookie sheets. Shape or roll dough into balls. Bake in a 350 degree F oven for 10 to 12 minutes or until tops are set (may appear underbaked). Cool on cookie sheets for 2 minutes. Transfer to wire racks and let cool.
Frost cookies with Chocolate Frosting. Makes 24 cookies.
In a small saucepan, melt butter over low heat, stirring constantly. Add cocoa powder and stir until combined. Remove from heat. Slowly beat in powdered sugar, vanilla and enough milk (1 to 2 tablespoons) to make a creamy consistency. Makes about 1 cup frosting.