Preheat the oven to 400°F
Line 12 muffin cups with paper liners or spray generously with nonstick cooking spray. Grease the top of the muffin pan.
For the streusel, mix all the ingredients together with a fork or your fingertips in a small bowl to make a crumbly mixture. Set aside.
For the batter, whisk the oil, brown sugar, zest, and egg together in a large bowl. The mixture should be pasty.
Stir in the vanilla and buttermilk.
In a large bowl, stir together 2 1/2 cups of the flour, the salt, baking powder, baking soda, and cinnamon. Add to the wet ingredients and blend.
Fold in the fruit. The use of frozen fruit may help firm up the batter. If not, and the batter seems too loose, add 1/4 cup more flour to make the batter a bit stiffer.
Using a large ice-cream scoop, fill the muffins cups to the top. Sprinkle streusel topping evenly over each muffin. Place the muffin pan on the prepared baking sheet.
Bake for 15 minutes, then lower the oven temperature to 350°F and bake for another 12 to 15 minutes, or until the muffins spring back when pressed lightly.
Let cool in the pan for 5 minutes before unmolding onto wire racks.