Buttermilk custard, buttermilk pie is an old southern tradition. I grew up eating the various types of these desserts.
Here is one
and another
Some that are similar are made with "clabbered" milk which requires raw milk to start. A good substitute is, if
you can find pasteurized milk that is not ultra-pasteurized, like Whole Foods and Trader Joes carry.
Is heated and cooked with lemon juice so it starts to form curds but instead of straining out the curds to make fresh cheese, you stir them back into the whey with a whisk and add 1/4 cup of heavy cream for each quart of milk.
You can hold this for 2-3 days in the fridge before using in your recipe.