Mix the flour, sugar, sourdough powder and yeast in a bowl. Add the milk, mix together, leave to absorb for approx. 1 hr.
Add the flour to the starter dough along with all the other ingredients up to and including the salt, knead into a soft, smooth dough using a food processor for approx. 15 mins. on a low setting. Add the butter and continue to knead for approx. 5 mins. Transfer the dough to the prepared tin, cover and leave to rise for approx. 2½ hrs.
Spread the knobs of butter on top of the dough, bake for approx. 75 mins. on the bottom shelf of an oven preheated to 160°C.