½ cup pecans
¼ cup (1¾ ounces) dark brown sugar
1 teaspoon ground cinnamon
2 cups (10 ounces) all-purpose flour
1 cup (7 ounces) granulated sugar
1 teaspoon salt
8 tablespoons (1 stick) unsalted butter, cut into ½-inch pieces and softened
1½ teaspoons baking powder
½ teaspoon baking soda
¾ cup sour cream
1 large egg
1 teaspoon vanilla extract
For Glaze (optional):
1 cup confectioner's sugar
2 tablespoons water
1. Adjust an oven rack to the middle position and preheat the oven to 350°F. Grease a standard 12-cup muffin tin and set aside.
2. In the bowl of a food processor fitted with the metal blade, process the nuts, brown sugar, and cinnamon until the nuts are the size of sesame seeds, about ten 1-second pulses. Use a rubber spatula to scrape down the sides and bottom of the food processor bowl and transfer the mixture to a medium bowl and set aside.
3. Return the bowl and metal blade to the food processor, add the flour, granulated sugar, and salt and process until combined, about five 1-second pulses. Sprinkle the butter evenly over the flour mixture and process until the butter is the size of oats, about eight 1-second pulses. Remove ½ cup of the flour-butter mixture and stir it with a fork into the reserved brown sugar mixture until combined to make the streusel. Set aside ¾ cup of the streusel for the muffin batter and the remaining portion for topping the muffins.
4. Add the baking powder and baking soda to the remaining flour mixture in the food processor bowl and process until combined, about five 1-second pulses. Whisk together the sour cream, egg, and vanilla in a 1-cup glass measuring cup and add to the flour mixture. Process until the batter is just moistened, about five 1-second pulses. Add the remaining ¾ cup of the streusel to the flour mixture and process until the streusel is just distributed throughout the batter and the batter looks crumbly, about five 1-second pulses.
5. Using a large spoon sprayed with nonstick cooking spray to prevent sticking, divide the batter among 12 muffin cups. Sprinkle with a scant tablespoon of streusel on top of each muffin, pressing lightly so that the streusel sinks slightly into the batter. Bake the muffins until a toothpick inserted in the center of a muffin comes out with several crumbs clinging to it, about 18 minutes, rotating the pan from front to back halfway through the baking time. Avoid overbaking. Place the muffin tin on a wire rack and allow the muffins to cool in the tin for 2 minutes. Using the tip of a paring knife, loosen the muffins and gently transfer them from the tin to the wire rack. Cool for 5 minutes. Serve warm.
6. If glazing the muffins, place a sheet of parchment paper beneath the wire rack as the muffins cool. Whisk the confectioner's sugar and water in a medium bowl until smooth. Spoon about 2 teaspoons glaze over each muffin, letting the glaze run down the sides of the muffins. Serve warm.