Prepare ice cream machine according to the manufacturer’s instructions. Be sure to freeze the bowl ahead of time if required!
Wash the rhubarb and trim its base and stems, if necessary. If your rhubarb has a clean cut on each end, there is no need to trim, but if the ends have browned or the stems are a bit green, you’ll want give them a clean break to remove these parts.
Chop the rhubarb into one-inch pieces, stir together with sugar in a medium-sized pot, and let sit at room temperature for one hour to macerate.
Cook the rhubarb, sugar, and all its juices on low heat, stirring regularly, until the chunks of rhubarb have disintegrated.
Chill over an ice bath for one hour, stirring every fifteen minutes.
Blend the rhubarb with the buttermilk in a blender, food processor, or with an immersion blender.
Conduct an egg test: place a clean, dry egg in the base to test its sugar ratio. If the egg sinks, remove it and blend in additional sugar 1 tablespoon at a time until the egg stays afloat.
Churn according to the manufacturer’s instructions.