This is one of my favorite ways to use up a little bit of leftover mashed potatoes. The potatoes make the bread very soft and moist.
Heat milk and butter in a small saucepan until butter melts. Stir in mashed potatoes. Dissolve yeast in lukewarm water.
In a mixing bowl, blend milk mixture, yeast mixture, sugar, salt, and egg. Add whole wheat flour, one cup at a time. Stir in enough white flour to make a soft, kneadable dough. Knead dough for 10 minutes.
Let rise in a greased bowl covered with plastic wrap until doubled in size. Punch down and form into 2 loaves. Put into loaf pan, cover with plastic wroap, and let rise.
Bake in 375 oven for 45 minutes. Remove from pan and cool on wire rack.