Pain de mie (version 2)

I was looking for a a good recipe for "pain de mie" or sandwich bread. I had once another try (here) and it was OK, but I'm still searching for the perfect one.
The idea of redoing this type of bread came into my mind when I saw a video stating that cast iron tins are perfect for this type of bread because they cook evenly. I cannot confirm this hypothesis, and it can be seen even in the photo that there was no real difference between the 2 baked in cake tins and the one baked in cast iron tin. On the contrary, the one baked in cast iron (top right) was difficult to take out and I had to scrub a bit to clean it, while the ones put in non-stick tins went out perfectly. I was expecting a more brown color on the bottom of the one made in cast iron. This still remains to be retested anyway.
In my rush of making this bread, I forgot one ingredient: butter. The original recipe was asking for 80g of butter in addition to the listed ingredients. It happened to me to measure the butter, to put it in a cup and microwave it for 20 second to melt it down. The problem was that I forgot the butter there and I realized this when  the loaves were already in the oven. Nevertheless the difference in taste or texture was not that big.
I am not a big fan of sweet breads, not that I dislike them, but let's say I prefer the salty ones with no sugar or milk inside. This one contains just a bit of them and it is acceptable. This bread ended up quite fine but to be honest I am still looking for the perfect recipe ...

Ingredients:

• 1000g bread flour (10% proteins)
• 560g water
• 150g liquid sourdough
• 10g fresh yeast
• 15g salt
• 20g powder milk
• 50g sugar
• 40g sour cream
• 1 egg (to brush the top)

Directions:

1. Mix flour with water for 4 minutes in a standing mixer and let it rest covered for the autolyse phase for 1h.
2. Add remaining ingredients (except egg) and mix 4 minutes on a low speed and 10 minutes on a higher speed.
3. Bulk fermentation 2h20" at room temperature.
4. Shape 3 loaves and put them in loaf pans that were initially buttered to avoid sticking.
5. Final fermentation, covered with a towel for 1h30 at room temperature. 
6. Brush the loaves with the beaten egg.
7. Preheat the oven at 170ºC and bake for 1h.
8. Let them cool 15 minutes in the loaf pan and take the loaves out by placing them on a rack, face down.

This recipe was inspired from Le Larousse du pain - Eric Kayser, page 210.