Vanilla and chocolate shortbread pinwheels

Method

1. For the vanilla shortbread, put all the ingredients in a mixing bowl with a pinch of salt and 1 tbsp cold water. Beat with a wooden spoon until combined. Tip out onto a work surface, shape into a rough rectangle, then wrap in cling film and chill for 1 hour.
2. Repeat this process for the chocolate shortbread, adding 1-2 tbsp extra cold water to give the mixture the same consistency as the vanilla dough. Shape into a rough rectangle, wrap in cling film and chill for 1 hour.
3. Heat the oven to 180°C/160°C fan/gas 4. Unwrap both types of dough, leaving them on the cling film, then lightly dust with flour. Roll each out to a rough 30cm x 20cm rectangle. Brush the top of the vanilla shortbread with a little milk and carefully but quickly flip the chocolate dough on top. Leave the cling film on and lightly roll over with a rolling pin to press the two doughs together.
4. Remove the top layer of cling film and, using a sharp knife, trim the edges to make one large, even piece of dough.
5. Using the bottom piece of cling film to help you, roll up the dough into a tight spiral and smooth with your hands to secure. Chill for 30 minutes (see Make Ahead).
6. Remove the cling film and brush the outside of the rolled dough with a little more milk. Scatter the work surface with the barista sugar, then roll the dough over the crystals to fully coat it. Slice the dough into 22-25 x 1cm-thick circles. Lay each on its side on the baking sheets, spacing them out, then bake for 15-18 minutes until lightly golden. Transfer the pinwheels to a wire rack to cool completely.