Streusel: In a medium bowl, mix together cinnamon, salt, brown sugar, and flour. Cut in butter with a pastry blender or fork until mixture forms fine crumbs. Using your hands, squeeze together most of mixture to form large clumps. Refrigerate in an airtight container until ready to use (up to 3 days).

Cake: Preheat oven to 350 degrees. Butter a 10-inch springform pan; dust with flour, tapping out excess. In a medium bowl, whisk together flour, baking powder, and salt.

In the bowl of an electric mixer fitted with the paddle attachment, beat together butter and granulated sugar on medium speed until pale and fluffy, about 3 minutes. Reduce speed to low; add egg and vanilla. Add flour mixture in 2 batches, alternating with the milk and beginning and ending with the flour; beat until just combined. Stir in berries.

Transfer batter to prepared pan; sprinkle topping over batter. Bake until a cake tester comes out clean, about 60 minutes. Transfer to a wire rack to cool 10 minutes. Run a knife around edge of pan to loosen. Remove side of pan. Let cool 15 minutes before serving.