Preheat oven to 375 degrees, F. Lightly grease muffin pans.
Beat butter at medium speed with an electric mixer until creamy; gradually add 1/3 cup sugar, beating until light and fluffy. Add egg, beating well.
Combine 2-1/2 cups flour, baking poweder and salt; gradually add to butter mixture alternately with milk, beginning and ending with flour mixture and beating well after each addition. Stir in vanilla, and fold in bludberries. Spoon batter into prepared muffin pans, filling two-thirds full.
Prepare Streusel Topping:
Combine 1/2 cup sugar, 1/3 cup flour, and 1/2 t. cinnamon; cut into 1/4 cup butter with a pastry blender until crumbly. Sprinkle on top of muffin batter
Bake at 375 degrees , F. for 35 minutes or until golden brown. Remove from pans immediately.
Alternate: Trade the blueberries for raspberries or blackberries for a change.