Ingredients

Streusel Topping:
1/3 cup brown or white sugar
1/2 teaspoon ground cinnamon
1 tablespoon unsalted butter
1/2 cup finely chopped walnuts

Muffins:
2 1/2 cups unbleached all-purpose flour
1 teaspoon baking soda
2 teaspoons baking powder
1/2 teaspoon ground cinnamon
Pinch of salt
1 1/3 cups light brown sugar, packed firm
2/3 cup vegetable oil
1 tablespoon minced citrus zest (from 1 lime, lemon or orange)
1 egg
1 cup buttermilk
2 teaspoons vanilla extract
1 3/4 cups fruit, lightly packed

Topping: Mix sugar and cinnamon in a small bowl or workbowl of a food processor; add butter. If mixing by hand, use fingertips, a pastry blender or two forks to blend the fat into dry ingredients until mixture looks like coarse irregular crumbs, with no visible lumps of fat. If mixing in a food processor, pulse about 10 times, then process five to 10 seconds until there are no visible lumps of fat; stir in nuts and set aside.

Directions

Muffins: Adjust oven rack to middle position and heat oven to 400 degrees. Lightly grease the top surface of a 12-cup muffin pan; use liners, if necessary. In a medium bowl, whisk 2 1/2 cups flour with baking soda, baking powder, cinnamon and salt; set aside.

In a large bowl, whisk together brown sugar, oil, zest and egg; whisk in buttermilk and vanilla. Gently whisk dry ingredients into wet ingredients to partially blend. Continue mixing batter with a rubber spatula, making sure that ingredients at the bottom are incorporated into batter; fold in fruit. (Frozen fruit will help "firm" up batter. If batter seems too wet, add a few more tablespoons of flour — up to 1/4 cup.)

Using an ice-cream scoop, place a portion of batter into each muffin cup, filling to the brim. Sprinkle a portion of streusel topping over batter in each muffin cup. (If you find you have unfilled muffin cups in your baking pan, fill those with water to distribute heat evenly).

Bake 15 minutes; reduce heat to 350 degrees and bake until muffins are golden brown and spring back when lightly pressed with fingertips, 10 to 12 minutes. Let muffins cool in pan for 5 minutes then transfer them to a wire rack. Serve warm or at room temperature. Makes 1 dozen.