Beat butter, powdered sugar, brown sugar and salt in large bowl with electric mixer at medium speed 2 minutes or until light and fluffy. Gradually add flour, beating well after each addition. Reserve one half of dough. Add melted chocolate and cocoa to remaining dough; beat until well blended.

Divide the chocolate and plain doughs each into 4 pieces; shape each piece into ball. Roll one ball plain dough into 12x6-inch rectangle on floured surface; transfer to sheet of parchment paper or plastic wrap. Roll one ball chocolate dough into 12x6-inch rectangle on floured surface; place on top of plain dough rectangle. Tightly roll up jelly-roll style, starting at the wide end, to form a 12-inch log. If dough crumbles or breaks, press back together and continue to roll. Wrap in plastic wrap; refrigerate 1 hour. Repeat with remaining dough.

Preheat oven to 300°F. Cut each log into 20 slices; place on ungreased cookie sheets. Bake 13 minutes or until set. Cool on cookie sheets 5 minutes. Remove to wire racks; cool completely.