1. Preheat oven to 350º. Butter a 8 or 9 inch baking pan that is at least 2 inches deep. Set aside.
2. For the Streusel: In a mixing bowl combine the sugar, flour, cinnamon and salt. Add in the butter and mix in with your fingers or two forks until it resembles crumbs. Set aside.
3. In a large mixing bowl cream together the butter and sugar. Add in the egg and vanilla, mix well.
4. In a separate bowl combine the flour, baking powder and salt.
5. Alternatively add the flour mixture and milk to the butter mixture, beginning and ending with the flour mixture. Stir in the blueberries.
6. Pour the batter in the prepared pan. Top with the cinnamon streusel mixture.
7. Bake for 40 minutes or until a toothpick or knife inserted into the center comes out clean. Cool completely. Store in an airtight container for 3 days at room temperature or freeze for up to 2 months.