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.

In a medium bowl, combine flour, sugar, cinnamon and salt. Using a pastry blender or fork, cut in the butter until fine crumbs form. Using hands, squeeze together most of the mixture to form large clumps. The topping can be frozen in an airtight container for up to 6 months. Makes enough to top 1 10-inch round cake or 2 6-inch round cakes.