Remove the top parchment and cut each rectangle into 3 equal strips, cutting across the long edge. Place 3 strips of dough on top of each other, alternating with vanilla and chocolate dough so you end up with 2 long slabs of dough, 3 strips high. Cut each slab again long ways into 3 equal strips. Remove the centre strip from each slab and place into the other slab so you have 2 slabs forming 9 small alternating rectangles cross-wise. Wrap each slab in cling film and chill for another 30 mins.