From America's Test Kitchen
Merely replacing butter and cream with buttermilk to create tangy, creamy buttermilk mashed potatoes doesn’t work—the finished potatoes are curdled, crumbly, chalky, and dry. We wanted easy mashed potatoes with buttermilk’s trademark distinctive tang, but we didn’t want to sacrifice texture to get them.
Many recipes for buttermilk mashed potatoes remove so much butter that the potatoes taste lean and lack creaminess. We started by restoring just enough butter to save our mashed potatoes from this fate. We then tackled the curdling problem. Buttermilk curdles at 160 degrees, a temperature reached almost instantly when the cold liquid hits steaming-hot potatoes. By adding the butter, melted, to room-temperature buttermilk, we coated the proteins in the buttermilk and protected them from the heat shock that causes curdling. We also simplified the recipe by choosing peeled and cut Yukon Gold potatoes rather than using unpeeled russets (our usual choice for mashed potatoes). Because Yukon Golds have less starch and are less absorbent than russets, they don’t become soggy and thinned out when simmered without their jackets.
To achieve the proper texture, it important to cook the potatoes thoroughly; they are done if they break apart when a knife is inserted and gently wiggled. Buttermilk substitutes like clabbered milk do not produce sufficiently tangy potatoes. To reduce the chances of curdling, the buttermilk must be brought to room temperature and mixed with cooled melted butter.