Cooling? Whatever for? I just drain them and mash them while they're still hot. Skin on.
Yeah. I've heard of cooling cookie dough in the refrigerator over night before baking, but this is the first I've heard of doing it to potatoes for mashing.
Jeff, I'll bet the potato water has a bit of starch, which probably helps to thicken a gravy. Nice idea.
Yup, I'm sure it's starch from the potatoes. Mother may even have said as much; I don't remember for sure. I
do remember that she said she learned to do that from my grandmother/her mother, and it wouldn't surprise me at all if Grandma learned it from my great-grandmother/her mother. They were all basically rural women, and that's how cooking was learned, from mother to daughter.
Mother and Grandma also always included a small piece of pork when they made a roast of beef. They said the mixed beef and pork drippings made a better tasting gravy.
I've never made mashed potatoes with the skins on, but, here again, I'm cooking like the women before me!
They never had fancy potatoes like Yukon Gold, just whatever regular old potatoes with thick skins that had to be peeled for cooking.