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 un-greased
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 200C/400F/Gas 6. Dust the
loaves with flour and bake until browned and the
bases sound hollow when tapped, 45-50 minutes. Cool on a wire rack.