Preheat oven to 375°. In a bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Add egg; mix well. Combine dry ingredients; add to creamed mixture alternately with milk. Stir in vanilla. Gently fold in blueberries. , Fill greased or paper-lined muffin cups two-thirds full. For streusel topping, in a small bowl, combine the sugar, flour and cinnamon; cut in the butter until crumbly. Sprinkle over muffins. Bake until a toothpick comes out clean, 20-25 minutes.