Substituting buttermilk in Pain de Mie

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Last week I made Pain de Mie for the first time, trying out my new Pullman loaf pan, and it worked really well.  I used this recipe:

http://www.food.com/recipe/pain-de-mie-french-pullman-bread-abm-303501

 except I used leftover mashed potato and added about 1/4 cup mixed grains for texture.  For the next loaf, I was thinking to try buttermilk instead of 2%.  Should I?  If I do, should I reduce the amount of butter in the recipe? 



Hi, I just bought two 9" pullman loaf pans, and am having a difficult time finding recipes for this size. I hadn't realized that the 13" size was the standard.  I tried scaling back one recipe, but the results were dry and not notably tasty. What I'm wondering is if I make the full-size recipe, then cut a third off the proofed dough (and presumably baking off that fragment as maybe a largish bun or smallish free-standing loaf), would I get the desireable results? Off-hand, I can't see why I wouldn't, but I may be overlooking some critical detail.