In a medium bowl, sift together flour, baking powder and salt. Set aside. In the bowl of an electric mixer, cream butter and sugar on medium speed until light and fluffy. Reduce mixer speed to low, and add egg and vanilla. When combined, alternate adding reserved flour mixture and milk to the mixer, beginning and ending with the flour mixture. Remove from mixer. Gently fold in blueberries. Pour batter into prepared pan. To make streusel topping, mix cinnamon, salt, brown sugar and flour together; cut in butter using a pastry blender or fork until fine crumbs are formed. Using hands, squeeze together most of the mixture to form large clumps. Sprinkle topping generously over batter.