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 and add cocoa to one portion. Divide each portion into half.

On a baking sheet, roll each portion between waxed paper into a 12×10 inch. rectangle. Refrigerate for 30 mins.

Remove waxed paper. Place one chocolate rectangle over a plain rectangle. Roll up tightly, jelly roll style. Starting with along side. Wrap in plastic. Repeat with the remaining dough.

Refrigerate for 2 hrs or until firm. Unwrap and cut into 1/4 inch slices.

Place 2 inch apart on a lightly greased baking sheet.

Bake at 350 degrees for 12 mins or till set.