Mix 100 g of flour with sugar, water, milk and dry yeast in a bowl and cover and let the pre-dough rise in a warm place for about 20 minutes.
In the meantime, peel the potatoes and press them through a potato press. Melt the butter and let it cool.
Stir the risen pre-dough. Mix in the potatoes, remaining flour, salt and fat and knead the dough until smooth. Let the ball of dough rise for about 1 hour until its volume has doubled. Knead again, shape into a loaf and let rise for another 40-50 minutes.