In a small/medium sized mixing bowl, cream together the butter and sugar. Add the vanilla and egg, and then add the flour until you have a fairly stiff dough. Wrap in plastic and refrigerate a minimum of 2 hours, or overnight.

In a small/medium sized mixing bowl, cream together the butter and sugar. Add the vanilla and egg, and then add the flour and cocoa until you have a fairly stiff dough. Wrap in plastic and refrigerate a minimum of 2 hours, or overnight.

Divide each kind of dough into 8 sections. Roll each section into an 18" log (about 1/4" thick) on a surface dusted with a bit of icing sugar or flour (icing sugar will help the layers stick together better once the pattern has been created).

Press two ropes of each kind of dough alternately beside each other (chocolate, vanilla, chocolate vanilla). Brush the strip with a little bit of water or egg white (optional) to help the next layer stick.

Cut the long strip of striped dough in half, turn one side over, and set it on top of the first section with the pattern alternating. Press together lightly, cut in half again, and stack the halves (with a tiny bit of egg white or water between the layers), making sure the pattern is still correctly lined up. Press the dough firmly into a square log (a bench scraper helps with this).

Repeat with the other strips of dough until you have 4 square logs. Wrap the logs in plastic and refrigerate overnight.

Cut the logs into 8-9 slices each (about 1/4" thick), and bake the cookies on parchment lined baking sheets at 350 degrees F for 8-10 minutes or until done. Cool on wire racks.