Directions

1. Line 2 x 900g loaf tins with cling film. Start by making the vanilla dough. Rub the flour and the butter together with your fingers until it looks like fresh breadcrumbs.
2. Mix the egg and the sugar together in another bowl with a fork and when it is really well combined pour it onto the flour mixture. Add vanilla extract, cream of tartar and a good pinch of salt. Knead together with your hands until the mixture forms a smooth dough. Press the block of dough into one of the prepared tins, press it down really well with the back of a spoon or another loaf tin and cover with cling film. Set in the fridge for 30 mins.
3. Make the chocolate dough in the same way, rubbing the flour, cocoa and butter together before adding the other ingredients with a pinch of salt, knead well and press into the remaining tin and set it in the fridge with the vanilla dough.
4. Once both the doughs are firm take them out of the tins, remove the cling film and trim the blocks so they both have straight sides.
5. Cut each dough block into 8 long slices along the length, then cut each of those slices into 3 strips. Stack the strips alternating to create a chessboard pattern, 4 pieces wide and 6 high - a cut through should be about the size of a playing card. Repeat the same process with the rest of the strips to give you 2 blocks of dough. Wrap both blocks really tightly in cling film, knock the sides on the work surface to give compact, flat sides. Return to the fridge to firm up for another 20 mins.
6. Heat oven to 160C/140C fan/gas 3 and line 2 baking sheets with baking parchment. Take the dough blocks out of the fridge, slice into 0.5cm slices. Lay the biscuits on the baking sheets and bake for 15-17 mins or until they are very lightly golden at the edges. You should be able to get about 9 or 10 on a baking sheet so you'll need to bake them in batches. Transfer to a wire rack and leave to cool while you bake the rest of the biscuits. If you want to trim the edges or cut out a shape from the centre do so just after they come out of the oven but before transferring to a wire rack (we used a mixture of heart, diamond, club and spade cookie cutters to create a fun playing card design).