To make the streusel, by hand, with a fork or pastry blender, mix the brown sugar, flour, cinnamon and butter until crumbly and well blended.
Preheat oven to 375*F. LIne 12 muffin cups with paper cupcake liners, or coat with melted butter and dust with flour, or spray generously with cooking spray. To make sure the muffins release with ease, coat the top of the pan as well as the inside of the cups when you grease the tins.
Sift together the flour, baking powder, baking soda and salt; set aside. Whisk the butter and brown sugar in a large bowl, then whisk in the eggs. Add the bananas, milk and vanilla. Add the dry ingredients all at once, stirring just until blended. Quickly fold in the berries. Divide into prepared muffin pans and sprinkle with 1/4 cup of the butter streusel (put remainder of the streusel in a jar, cover and refrigerate for another use, it's very good sprinkled on all kinds of muffins).
Bake 25 to 30 minutes or until the tops of the muffins are golden brown and they spring back when lightly touched with a finger. Cool for about 5 minutes, then remove from the pan.