In a large mixing bowl place all the flours, salt, yeast and butter together.
Add half of the water and mix everything together well.
Add the remaining half of the water and mix everything together to form a dough ball.
Lightly dust a clean surface with some flour and knead the dough ball for about 3 – 4 minutes on this surface. Then place the dough ball back into the large mixing bowl and cover the bowl with cling film. Leave to rest for about 2 hours.
After 2 hours, knead the dough a little, then shape the dough into a ball and place it onto a baking tray. Place the tray with the dough on it into a loose fitting bag and allow to rest and rise again for 1 hour.
Pre-heat the oven to 220C Fan.
Using a sharp knife carefully make some diagonal slashes across the top of the dough, dust with some flour and bake in the centre of the oven for about 30 minutes until dark brown and hollow sounding when tapped.
Allow to cool on a wire rack for at least 45 minutes to 1 hour before cutting and serving.
Freeze any leftover slices on day of making. Makes approximately 12 slices.