Heat oven to 350 degrees. Butter a springform baking pan and dust with flour, tapping out excess. Set aside. In a medium bowl, sift together the flour, baking powder and salt; set aside. In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugar on medium speed until light and fluffy, about 3 minutes. Reduce mixer speed to low, and add egg and vanilla, beating until fully combined. Add reserved flour mixture, alternating with the milk, a little of each at a time, starting and ending with the flour mixture. Remove from mixer. Gently fold in blueberries. Pour batter into prepared pan; sprinkle streusel topping over cake. Bake until cake tester comes out batter-free, 60 to 70 minutes. Transfer to a wire rack to cool for 10 minutes. Remove from pan; cool for 15 minutes before serving. Makes 1 10-inch round cake or 2 6-inch round cakes.