Blueberry Streusel Muffins

In a large bowl, cream butter and sugar. Beat in egg and vanilla; mix well. Combine the flour, baking powder and salt; add to creamed mixture alternately with milk. Fold in blueberries. , Fill 12 greased or paper-lined muffin cups two-thirds full. In a small bowl, combine the sugar, flour and cinnamon; cut in butter until crumbly. Sprinkle over muffins. Bake at 375° for 25-30 minutes or until browned. Cool for 5 minutes before removing to a wire rack. Serve warm.