I've made milk soaps a couple of times without any problem. I just weighed out my milk and put in ice cube tray, froze it to soap the next day. When ready to soap, I just put the cubes in a big bowl, pour in my lye (not slowly, just all at once works for me), and then stir and stir for about 5-6 mins. It melts the cubes up fine and dissolves the lye. At that point my lye solution gets about 80-90 degrees, then I just add to my fats and proceed as normal.