Cut four 14 inchx12 inch pieces of waxed paper or plastic wrap; set aside. In a bowl, cream butter or margarine, sugar, eggs, and vanilla until light and fluffy. In a bowl, combine flour, baking soda and cocoa powder. Stir flour mixture into sugar mixture until evenly distributed. Divide dough into 4 pieces. Shape each piece into and 8 to 25 cm roll. Wrap each roll in waxed paper or plastic wrap. Place in freezer container with a tight-fitting lid, or wrap airtight in a piece of foil. Label and date. Store in freezer. Use within 6 months.
To make cookies: Slightly thaw a roll of dough. Preheat oven to 180ºC . Lightly grease baking sheets. Slice dough into slices or chunks. Arrange slices on prepared baking sheets about 1 cm apart. Bake 8 to 10 minutes until cookies are set on edges and slightly firm on top. Remove cookies and cool on wire racks.