Divide dough in half. Mix cocoa into one half of dough.
Pat or roll each half into 9-inch square. I recommend doing so on waxed paper. Cut each square into 3-inch strips. Cut strips crosswise into halves.
Lift 1 brown strip with large metal spatula onto plastic wrap. Top with 1 white strip. Press firmly together. Top with remaining strips, alternating colors and pressing firmly to form a bar. When finished the bar should be about 4 1/2 inches long, 3 inches wide, and about 3 inches high.
Wrap bar in plastic wrap and refrigerate until chilled, 1 to 2 hours.
Heat the oven to 375 degrees. Cut bar into quarter-inch thick slices. Cut each slice crosswise into halves. Place on ungreased cookie sheet.
Bake cookies for 8 to 10 minutes, or until edges begin to brown.