Preheat your oven to 375 degrees. Butter twelve muffin cups.
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, and 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.
Do ahead: Generally, I think muffins are best on the first day, but these surprise me by being twice as moist, with even more developed flavors, on day two. They're just a little less crisp on top after being in an airtight container overnight.
NotesCook's Note: You don't create seven muffin recipes in a year without learning a few things. I found that you could dial back the sugar in most recipes quite a bit and not miss much (though, if you find that you do, a dusting of powdered sugar or a powdered- sugar- lemon- juice glaze works well here); that a little whole- wheat fl our went a long way to keep muffins squarely in the breakfast department; that you can almost always replace sour cream with buttermilk or yogurt, but I like sour cream best. Thick batters- batters almost like cookie dough- keep fruit from sinking, and the best muffins have more fruit inside than seems, well, seemly. And, finally, in almost any muffin recipe, olive oil can replace butter, but people like you more when you use butter- and if you brown that butter first, you might have trouble getting them to leave.