Hi Davidsoaps,
I made a yogurt soap Monday night, and what I did was measure the amount I needed (I used half distilled water/half yogurt as my liquid), freeze it in an ice cube tray, and then the next day, I took it out and put it in a stainless steel pot. I put the pot inside a glass baking dish and added water and ice cubes to the baking dish so that the pot sat in an ice water bath. I added the lye just a little bit at a time, stirring until each bit was dissolved into the yogurt. I monitored the temperature, and it stayed low the whole time and didn't scald. I actually had to put in into a warm water bath (removed cold ice water from the baking dish and added hot tap water) to bring the temp up to 90º. It might have been an unnecessary step, but I was working with just olive and coconut oils, and my experience was that those oils alone took a long time to saponify.