She hasn't said so in so many words, but I have a hunch that my editor thinks I should explain why it took me no fewer than seven muffin recipes to stop fussing and find the perfect one to tell you about. Are muffin recipes that hard to come up with? No, not really. Do we perhaps just enjoy eating muffins so much that I looked for excuses to make more? Unfortunately, not that either. Am I really so terribly indecisive? Apparently, yes, but only in what I believed to be the quest for the greater muffin good. Okay, fine, and when I'm choosing earrings. What finally led me here was, innocently enough, a basket of boring- looking lemon- poppy seed muffins at a bakery one morning; they got me wondering when poppy seeds would come untethered from lemon's grasp. Poppy seeds are delightful on their own- faintly nutty bordering on fruity- but they also play well with fruit that is richer in flavor and texture than lemon. Inspired, I went home and, a short while later, finally pulled a muffin out of the oven I'd change nothing about. Poppy seeds, plums, browned butter, brown sugar, and sour cream form a muffin that's rich with flavor, dense with fruit, and yet restrained enough to still feel like breakfast food. Seven rounds and six months in, I bet somewhere my editor is breathing a sigh of relief.
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.