Raspberry Streusel Muffins

In a mixing bowl, cream butter and sugar until light and fluffy; beat in egg. In a small bowl, mix sour cream, milk and vanilla. Combine dry ingredients; stir into creamed mixture alternately with sour cream mixture just until moistened. Gently fold in raspberries. Fill greased or paper-lined muffin cups two-thirds full. Combine flour, oats, sugar, cinnamon and salt; mix well. Cut in the butter until crumbly. Sprinkle over muffins. Bake at 400 degrees F for 18-22 minutes or until muffins test done. Cool in pan 10 minutes before removing to a wire rack. Dust win confectioners' sugar.