Danish Cold Potato Salad — "Kold Dansk Kartoffelsalat"

My mother-in-law makes the dressing for this potato salad using a product called Ymer, or A-38, or A-Tykmælk. For info on Ymer, see What the heck is Ymer? None of those ingredients listed above are available for purchase beyond the highly vigilant Customs police who patrol Denmark’s border. I’m not kidding. The Danes have even…