Heat milk until it starts bubbling on the sides. Remove from heat and add 50 g sugar, salt and butter. Mix until the butter has melted. Add mashed potatoes and allow to cool until lukewarm. Mix cake flour, whole-wheat flour and yeast. Add milk mixture and raisins and mix to form a stiff dough that doesn't stick to the sides of the bowl. Knead for about 15 minutes until smooth and elastic.
Place in an oiled bowl, cover and leave to rise in a warm place until double in volume.
Mix 50 g sugar with cinnamon and set aside.
Place dough on a floured surface and knock down.
Divide in two and roll out into two rectangles, each the size of a loaf tin.
Sprinkle half the cinnamon sugar over each rectangle and roll up.
Place in two greased loaf tins.
Brush lightly with melted butter, cover and allow to rest in a warm place for about an hour or until double in volume.
When risen, brush with remaining melted butter and bake for 40 minutes in a preheated oven at 180°C.
Turn out on a wire rack and sprinkle with some sugar.
Serve hot or cold.