Preheat oven to 400℉. Grease 12 2½-inch muffin cups (or six extra large). Set aside.
Prepare streusel topping: In a small bowl, combine flour, brown sugar, and cinnamon. Cut in butter until mixture resembles coarse crumbs, Stir in nuts. Place in freezer.
Prepare muffins: In a medium bowl, whisk together flour, sugar, baking powder, baking soda, salt and nuts.
In a 2 cup or larger measuring cup, combine banana, yogurt, eggs and melted butter.
Make a well in the center of the flour mixture. Pour liquid into the well. Stir liquid into flour until just combined.
Fill muffin cups ⅔ full. Crumble streusel mixture on top of each muffin.
Bake at 400℉ until golden, about 15 minutes. Put muffin pan on a cooling rack for 5 minutes. Remove muffins and serve while still warm.