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.
Excerpted from THE SMITTEN KITCHEN COOKBOOK by Deb Perelman. Copyright (c) 2012 by Deb Perelman. Excerpted by permission of Knopf, a division of Random House, Inc. All rights reserved.