Preheat the oven to 375°F. Grease a 12-cup muffin pan, or line with muffin liners.
In a large bowl, using an electric mixer, cream together the ½ cup unsalted butter ¾ cup of the 1 cup light brown sugar or dark brown sugar (reserving the remainder to sprinkle over the top) until fluffy. Beat in the 1 large egg, then on low speed mix in the 1 cup plain whole milk yogurt.
Stir in the 2 cups whole wheat pastry flour or oat flour, 1 teaspoon ground cardamom, 1 teaspoon baking powder, 1 teaspoon baking soda, and ¼ teaspoon fine sea salt. Gently fold in the prepared 2 pears and ¼ cup chopped walnuts.
Divide the batter evenly among the prepared muffin cups, then sprinkle the remaining ¼ cup of brown sugar evenly over the top.
Place in the oven and bake for 25 to 30 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove muffins from the oven, cool 5 minutes in the pan, then turn them out onto a rack to finish cooling completely.
Notes
Choose ripe but firm pears. You want pears that are sweet and fragrant but still firm enough to hold their shape when chopped and baked. Bartlett, Bosc, Anjou, or Comice pears all work well.
Don't overmix the batter. Once you add the dry ingredients, mix just until everything is incorporated. A few small streaks of flour are fine, but overmixing can make the muffins dense rather than tender.
Watch the baking time. The muffins are done when a tester inserted in the center comes out clean or with a few moist crumbs. Because pears vary in size and juiciness, your muffins may need a few extra minutes in the oven.