Grease two loaf pans measuring approximately 18cm x 10cm x 9cm.
Sift flour into the bowl of a food processor fitted with a dough blade or beater. Add the yeast and put on pulse for 1030 seconds. Mix in sugar, salt, milk powder, bread improver, egg and pour in 100ml water. Mix on a slow speed for 12 minutes. Gradually add in remaining water to mix until a dough is formed. (Add more water if the dough is too dry.)
Increase the speed to medium and continue to mix for another 12 minutes. Stir in butter and knead on a low speed for 23 minutes. Add in raisins and continue to knead for 35 minutes. Cover with a clean tea-towel and allow to rise in about 30 minutes.
Turn dough onto a lightly floured surface. Knead until smooth. Divide dough into two equal portions. Roll out each portion of dough to a rectangle then roll up from the short side like a Swiss roll and place into a greased loaf pan. Put the loaf pans in a large oiled plastic bag. Leave aside to rise in a warm place for about 30 minutes.
Use a sharp blade to cut a slit, lengthwise, across the centre. Put a piece of butter on the cut. Bake in preheated oven at 190°C for 2530 minutes until the loaf is golden brown and the base sounds hollow when tapped. Remove from oven and brush lightly with beaten egg yolk then turn out onto a wire rack to cool completely.