Preheat the oven to 350 degrees F (175 degrees C).

Beat butter or margarine and brown and white sugars together until creamy. Add eggs and vanilla; beat well. Combine flour, cocoa powder, baking soda, and salt in a separate bowl; add to butter-sugar mixture, stirring until well combined. Stir in oats and raisins; mix well.

Drop dough by rounded tablespoonfuls onto ungreased cookie sheets. Bake until cookies are almost set, 10 to 12 minutes. Do not overbake. Cool 1 minute on cookie sheets, then move to wire racks. Cool; store tightly covered.