This weekend, buy some buttermilk and some heavy cream. Find a large glass jar. Fill it halfway up with heavy cream. Add half again as much buttermilk. Stir. (Recipe note: 2 parts heavy cream to 1 part buttermilk. There was some confusion.)
Let it fester at room temperature. All weekend. Without a lid. Leave it in your windowsill or next to your stove or on your kitchen table. If there are any kids around, give them the job of stirring it a few times a day. And tasting it.
It will thicken. It will start to make your mouth pucker. Take it further than you think you should (in hot and humid weather, the souring process can happen very quickly so be vigilant). When you're pleased with the flavor, cover and store it in the fridge.