Combine all the starter ingredients in
a bowl. Cover and leave in a warm place for 2-3 hours
Place the flours, salt and butter in a
food processor and process just until blended, about 1-2 minutes.
Stir together the milk and starter,
then slowly pour into the processor, with the motor running, until the
mixture forms a dough. Knead until smooth.
Place in an ungreased bowl, cover with
a plastic bag, and leave to rise in a warm place until doubled in size,
about 1 1/2 hours. Knead again then return to the bowl and leave until
tripled in size, about 1 1/2 hours.
Grease a baking sheet. Divide the
dough in half. Cut off one-third of the dough from each half and shape
into four balls. Top each large ball with a small ball and press the
centre with the handle of a wooden spoon to secure. Cover with a plastic
bag, slash the top, and leave to rise.
Preheat the oven to 200oC /
400oF / Gas 6. Dust the loaves with flour and bake until
browned and the bases sound hollow when tapped, 45-50 minutes.