Preheat the oven to 170C/325F/Gas 3. Grease and line an 18cm/6in square tin with baking parchment.
For the vanilla dough, rub the flour, icing sugar, butter and vanilla extra together in a bowl until the mixture comes together as a dough.
For the chocolate dough, rub the flour, cocoa and butter together until the mixture comes together as a dough.
Roll out both doughs into a sqaure measuring 1cm/½in thick. Place the chocolate dough onto the vanilla dough and press firmly together. (Brush with some water to help the adhesion if required.) Cut the dough in half and then press these two pieces together to produce four layers of dough.
Cut into three even strips and then cut each strip at 5cm/2in intervals to produce biscuit-sized shapes. Place the dough into the baking tin and bake for 15-20 minutes.
Meanwhile, melt the chocolate in a glass bowl set over a saucepan of simmering water.