In a large bowl, cream butter and sugars until light and fluffy. Add the eggs, one at a time, beating well after each. Beat in vanilla. Combine the flour, baking powder and salt; gradually add to the creamed mixture and mix well. , Divide dough in half; add cocoa to one portion. Divide each portion in half. On a baking sheet, roll out each portion between waxed paper into a 12x10-in. rectangle. Refrigerate for 30 minutes. , Remove waxed paper. Place one chocolate rectangle over a plain rectangle. Roll up tightly, jelly-roll style, starting with a long side; wrap in plastic. Repeat with remaining dough. Refrigerate for 2 hours or until firm. Unwrap and cut into 1/4-in. slices. , Place 2 in. apart on lightly greased baking sheets. Bake at 350° for 10-12 minutes or until set. Remove to wire racks to cool.