Preheat oven to 375 degrees. Grease 12 muffin cups or line with paper liners.
Whisk the egg with both sugars in the bottom of a large bowl. Stir in the melted butter, then the sour cream. In a separate bowl, mix together the flours, baking powder, baking soda, salt, cinnamon, nutmeg, and poppy seeds, then stir them into the sour cream mixture until it is just combined and still a bit lumpy. Fold in the plums.
Divide batter among prepared muffin cups. Bake for 15 to 18 minutes, until the tops are golden and a tester inserted into the center of a muffin comes out clean. Rest muffins in the pan on a cooling rack for 2 minutes, then remove them from the tin to cool them completely.
Notes
To brown the butter: In a large pot, melt butter over medium-low heat. It will melt, then foam, then turn clear golden, and finally start to turn brown and smell nutty. Stir frequently, scraping up any bits from the bottom as you do. Don't take your eyes off the pot: You may be impatient for it to start browning, but the period between the time the butter begins to take on color and the point where it burns is often less than a minute.