Place Dr. Oetker Nona Tart Mix, corn starch, butter and egg in a large bowl, mix till dough forms.
Divide dough into two, add cocoa powder over one of the halves. Knead until cocoa powder has been fully incorporated.
Keep the dough in freezer for 30 minutes. Divide both dough again into 3 parts.
Roll each dough into square shape, about 1cm thick and cut into three slabs of equal width.
Arrange the dough slab in vanilla-chocolate-vanilla order and another one in chocolate-vanilla-chocolate order. Gently press the slabs together.
Cut each slabs dough into 3 strips of equal width. Rearrange the strips of dough to create 2 slabs forming 9 small rectangles cross-wise.
Keep in the freezer for another 10 minutes. Slice the dough about 1cm thick and place the dough on a lined baking pan.
Bake at 180°C for 15 to 20 minutes, or until lightly browned.