1In a bowl, using an electric mixer, beat butter and icing sugar together until pale. Stir in flour until mixture forms a dough.
2Divide dough into 2 equal pieces. Mix cocoa into one half and white chocolate into the other. Chill dough 30 mins.
3Roll both dough halves out between 2 sheets of baking paper to 0.5 cm-thick rectangles.
4Place white chocolate dough on lightly floured surface. Top with dark chocolate dough. Fold into thirds widthways, then in halves lengthways. Cut dough in half again. Shape each half into a log (about 6 cm in diameter). Wrap logs in plastic wrap and chill for 1 hour until firm.
5Preheat oven to moderate, 180 C. Line 2 oven trays with baking paper.
6Cut logs into 1 cm slices, arrange on tray 3 cm apart. Bake 12 to 15 mins until golden. Cool on trays 5 mins before transferring to a wire rack to cool completely. Store in an airtight container.