Preheat the oven to 375°F. Lightly grease an 8" square, 9" square, or 9" round cake pan. If you use an 8" square pan or round pan, make sure it's at least 2" deep., To make the streusel topping: Mix the sugar, flour, cinnamon, and salt in a small bowl. Cut or rub in the butter with the side of a fork, two knives or your fingertips until it reaches a crumbly state. Set aside., To make the cake: Blend the flour, baking powder, and salt together in a medium-sized mixing bowl., In a separate bowl, beat together the sugar, butter, egg, and vanilla., Alternately add the milk and the flour mixture to the sugar/butter mixture, ending with flour. Add blueberries. Stir only enough to blend., Pour the batter into the prepared pan. Sprinkle the streusel topping over the batter., Bake the cake for 40 to 45 minutes, or until a toothpick or knife inserted into the center comes out clean., Remove the cake from the oven, and set it on a rack to cool for 10 minutes. Serve right from the pan. Or, to transfer to a serving plate, Loosen the sides with a knife or spatula. Holding the pan in your left hand, gently tip the cake out onto your right hand, remove the pan, and gently right the cake onto a serving dish.