Mix sugar, cinnamon and pinch of salt in a small bowl or work bowl of a food processor, add butter. If mixing by hand, use fingertips, a pastry blender, or 2 forks to blend the fat into dry ingredients until mixture looks like coarse irregular crumbs, with no visible lumps of butter. If mixing in a food processor, pulse about 10 times, then process 5 to 10 seconds, until there are no visible lumps of butter; stir in nuts. Set aside.
For the 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.
Whisk 2 1/2 cups flour with next 4 ingredients in a medium bowl, set aside.
Whisk together next 4 ingredients in a large bowl; 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, to stiffen batter.).
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.
Bake 15 minutes, reduce heat to 350 degrees and bake 10 to 12 minutes or until muffins are golden brown and spring back when lightly pressed with fingertips. Let muffins cool in pan for 5 minutes then transfer them to a wire rack. Serve warm or at room temperature.
To simplify muffin making, quadruple the streusel topping recipe and freeze.
Orange, lemon, or lime zests can be used to flavor the muffins, if you like.
Lemon works well with blueberry.
Orange zest with cranberry or rhubarb.
Lime zest with banana chunks.
Almost any fruit works in this recipe. Good choices include:
apples, cut into small dice.
bananas, cut into firm, small chunks.
cranberries, coarsely chopped.
dried sour cherries or cranberries.
rhubarb, diced (fresh or frozen).
strawberries, quartered or cut into small dice.
To prevent delicate, highly colored fruits like raspberries, cranberries, and blueberries from getting mashed and discoloring the batter, use semi frozen fruit.
It is not necessary to use paper baking liners if the muffin tin is commercial, glazed, or nonstick.