I have just started this wonderful hobby, and have only made 7 batches... but have used 7 different recipes. All have been made with goat's milk except this last one. I use wooden molds that my husband made for me, and line them with parchment paper.Rebecca, what is your formula? I bet the good folks on here would have excellent suggestions for you.
Goat's Milk will contribute to overheating. I put all Goat's milk soap in the freezer, they are not hot enough to effect the freezer temp if put in immediately. You can also try chilling your mold, before pouring the batter
I haven't tried any "plain" batches yet... I was going to try an aloe soap next. The only additive being aloe... Maybe I can start with that...Milks, beers, honey, any additives with sugars will contribute to overheating. Have you tried doing a recipe without any additives, ie, just oils, lye/fragrance/colorant? That might be a good way to target what is leading to the overheating, you can add stuff from there and it will be easier to identify the problem factors if/when they arise.
That is called "glycerine rivers". Often times it happens with overheating AND the use of titanium dioxide (which in this case I assume you used to get that white color?) The oil sitting on top is also likely from overheating. Did you CPOP? Otherwise the sugars in beer can cause overheating as well.
ETA: Beautiful soap by the way.