From "The Smitten Kitchen Cookbook" by Deb Perelman
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 flour 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.
Plum Poppyseed Muffins
Makes 12 standard muffins
6 tablespoons unsalted butter, melted and browned and cooled, plus butter for muffin cups
1 large egg, lightly beaten
1/4 cup granulated sugar
1/4 cup packed dark or light brown sugar
3/4 cup sour cream or a rich, full-fat plain yogurt
1/2 cup whole wheat flour
1 cup all-purpose flour
3/4 teaspoon baking powder
3/4 teaspoon baking soda
1/4 teaspoon table salt
Pinch of ground cinnamon
Pinch of freshly grated nutmeg
2 tablespoons poppy seeds
2 cups pitted and diced plums, from about 3/4 pound Italian prune plums (though any plum variety will do)
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, 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.