Thanks to both Mr. DiMuzio and bblearner's input, I made my first pain de mie successfully.   This loaf was made with 20% pre-ferment maintained at 50% hydration. To add a bit of Japanese touch to the loaf,  3% flour and 17% water was used to make water roux starter. Honey was used in place of sugar.  The rest were milk, milk powder, butter and etc., pretty typical ingredients.

As Mr. DiMuzio mentioned, this sourdough sandwich loaf is more compact than the yeasted version loaves and has more substance (bigger dough size) and is slightly chewier in texture.    It is a nice loaf of milky savory bread with a very subtle hint of tang. 

My kids were fascinated with the square shape and called it the loaf with no 'butt' (as compared to the dome-shaped loaves I made before).  They've enjoyed it without butter and have already requested another loaf.

The kneading was done completely by my Zojirushi and no folds by hand.  I adopted Mr. DiMuzio's procedures to utilize the fridge to both bulk ferment and proof.  The loaf was done with minimal dough-sitting, which was my ultimate goal.   I'd also applied the same procedures in my nut-and-fruit bread and it turned out great as well.  Thank you again, Mr. DiMuzio.

http://www.flickr.com/photos/33569048@N05/sets/72157622439989640/