Buttermilk didn't make this cake* pretty. The bundt pan did that.
What did the buttermilk do? From my experience with cake recipes (like devil's food) using sour cream or buttermilk, I know these ingredients are acidic and will contribute slight tangy flavor and tenderize a cake's structure. However, I am careful about substituting acidic ingredients in recipes, especially to recipes with downy soft cake flour, because I am easily heartbroken by a fallen cake. One doesn't want to tenderize to the point of collapse. (I suppose that's true of people...and not just cakes!)
I still lack a solid foundation in how this works (and the vocabulary to match), but I know that the addition of buttermilk to a recipe using all-purpose flour will be softer that if I used regular milk. Well a knowledgable baker via Rose Beranbaum's forums explains it better that I: "buttermilk is acidic...which weakens the gluten in the flour, and makes all-purpose flour behave more like cake flour in terms of gluten strength." So we are weakening gluten! that explains why some of my cakes and cupcakes have fallen (teardrop). But does buttermilk only affect structure?
In Rose Levy Beranbaum’s recipe for blueberry muffins , she recommends that, when using buttermilk, to substitute baking powder for some of the soda for flavor...not for texture. I wonder why that is?
I am doing more research on buttermilk to find out.
*I made this recipe (Bon Appetit, April 2004) exactly as written and it was promptly devoured, so there is no adapted version to post. However, I found the finished cake ever-so-slightly sweet for my taste, and were I to make it again, I would tweak it as follows:
-Use frozen wild blueberries (and not fresh) for better distribution throughout each cake slice. The recipe already specifies frozen, but wild are smaller, more instensely flavored and will not get in the away of creating a soft crumb.
-I would use less sugar (probably 1-1/3 cups?) in the cake batter and make a small amount of tart glaze with freshly squeezed lemon juice to pour over the top. It would be pretty (bundt pans were made for glaze!) and add brightness to balance the soft and sweet cake.