In a large mixing bowl, cream butter and sugar. Beat in egg and vanilla; mix well. Combine flour, baking powder, and salt and add to creamed mixture alternately with milk. Fold in blueberries. Fill 12 greased muffin cups two-thirds full. In a small bowl combine the sugar, flour, butter and cinnamon until crumbly. Sprinkle over top of muffins and bake in a 375 degree oven for 25-30 minutes or until browned. Cool 5 minutes before moving to a wire rack.
A little note: I don not use paper liners. I grease muffin tins on the bottoms and only half way up the sides. The ungreased portion will allow the batter to climb up the sides for a higher muffin. (this hint came from Taste of Home).