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Tip the butter, caster sugar and flour into a food processor with a pinch of salt and blitz until the mixture looks sandy. Add the egg yolk, vanilla and 1 tbsp cold water, and blitz again until the dough starts to clump together.
Remove half of the dough from the food processor and set aside. Add a few drops of red food colouring to the dough still in the food processor and blitz until it’s bright red. Halve the reserved plain dough, then add a few drops of green food colouring to one piece, kneading it until it is evenly coloured.
Roll the red biscuit dough into a long sausage shape on a lightly floured surface – it should be about 20cm long. Roll the plain dough out on a lightly floured surface into a 20 x 10cm rectangle. Repeat with the green dough, rolling it out to the same size. Roll the plain dough around the red dough, bringing the ends together and trimming any excess. Repeat with the green dough, rolling it around the plain layer. Wrap the sausage of dough tightly and chill for at least 2 hrs. Will keep wrapped and frozen for up to two months.
Heat the oven 180C/160C fan/ gas 4 and line two baking trays with baking parchment. Unwrap the dough sausage, then use a sharp knife to cut it into rounds about 1cm thick. Arrange these on the prepared trays, then cut each round in half to make the watermelon wedges. You should have about 40 in total. Bake for 12-15 mins, swapping the trays over halfway through cooking if needed. Leave to cool completely on the trays.
Once cooled, pipe small dots over the red ‘watermelon flesh’ using the black icing pen to look like seeds. Leave to set for 30 mins before serving or packing into bags. Will keep in an airtight container for up to three days.