I found this recipe in a library book called A Passion for Baking by Marcy Goldman.
The recipe says it makes 9 to 12 muffins, but I assume those would be the jumbo size and not the traditional muffin pan, because I made about 20. Preheat oven to 400. Arrange rack in the middle. Line a 12 cup muffin pan with paper liners and place it on a parchment parer-lined baking sheet (I admit, my sheet was not parchment lined).
1/2 cup finely chopped walnuts or shredded coconut (I used coconut)
1/3 cup firmly packed brown or white sugar (I used brown)
1 tablespoon unsalted butter
1/4 teaspoon ground cinnamon
1/2 cup unsalted butter, melted
1 cup white sugar
1/2 cup firmly packed light brown sugar
1 tablespoon finely minced orange zest (I used a clementine to the same effect)
1 large egg
1 cup buttermilk (author says you can substitute sour cream, I used non-fat vanilla yogurt)
2 teaspoons vanilla
2 1/2 to 2 3/4 cups flour
2 teaspoons baking powder
1/2 teaspoon baking soda
1/8 teaspoon salt
1/8 teaspoon cinnamon
2 scant cups semi-frozen blackberries
Prepare streusel topping by rubbing or crumbling ingredients together with fingers; set aside.
In a large bowl whisk together melted butter, sugars, zest and egg. Do not overbeat. Mixture should be pasty. Add buttermilk and vanilla.
In a separate bowl, stir together flour through cinnamon. Gently fold into batter. Once partially blended, fold in fruit. Semi-frozen fruit helps 'firm up' the batter. If it seems too loose, add a few tablespoons of flour to make it stiffer.
Using a large ice cream scoop, scoop a generous amount of batter into each muffin cup. Sprinkle streusel topping evenly over each muffin. Bake 15 minutes. Reduce temp to 350 and bake another 12-15 minutes. Muffins should spring back when gently pressed. Allow to cool 5 minutes before removing from pan.