Set oven to 150°F Pour buttermilk into the baking dish and cover with tinfoil. Place the dish in the oven and let it sit overnight, or about 8-12 hours.
In the morning, remove the dish from the oven. Place a strainer in the sink or a large pot and line it with a kitchen towel or cheesecloth. Spoon or the contents of the baking dish into the strainer. Cover the cheese with the ends of the cloth and let it drain in the sink for 30-60 minutes.
Put the drained curd into a clean bowl and refrigerate. Once chilled, add milk a little (a tablespoon or two) at a time until it is smooth and easy to spread. You want a texture similar to ricotta cheese. Add salt to taste.