Cocoa and vanilla cookies

Method

1. In a food processor, mix the flour and butter until they resemble breadcrumbs. Add the icing sugar, egg, vanilla and 1 tbsp water, pulsing a few times until a dough forms.
2. Divide the dough into 2 equal parts. Return 1 to the food processor with the cocoa powder and cardamom and pulse briefly to mix together. Turn out the dough, form it into a ball, then flatten into a disc, wrap and put in the fridge.
3. Bring the other half of the dough together, forming it into a ball and flattening into a disc, then wrap it and put it in the fridge too. Chill both doughs for 10 minutes or until firm enough to roll while still being flexible.
4. On a lightly floured work surface, roll each dough out to a rectangular shape roughly 20cm x 25cm x 4mm thick. Trim the edges to even out, if necessary, so both are the same size. Brush the vanilla rectangle with some of the egg yolk glaze, then put the cocoa rectangle on top. Roll both layers up together like a huge cigar, then gently press-and-roll a few times to ensure both layers are sticking together. Wrap and put in the fridge for 1 hour.
5. Heat the oven to 160°C fan/gas 4. Line a baking sheet with baking paper. Unwrap the dough and cut the log into 6mm slices, laying them flat on the lined baking sheet. Bake for 15 minutes or until the cookies start to change colour slightly around the edges. Allow to cool and enjoy.