This is an variation on a recipe found in The Muffin Lady cookbook. I used plums, because that's what I had, but peaches would be delicious, too. She says to use unripe fruit, as it will ripen up and get sweet during cooking. Also, use sliced fruit rather than chopped, because chopped will tend to "get lost" during baking. These muffins are like a hand held fruit cobbler. So good!