Make the streusel: Stir together 1 cup flour, brown sugar, cinnamon, and 1/4 teaspoon salt. Cut in cold butter until mixture resembles coarse crumbs with pea-sized bits. Refrigerate while preparing batter.

Preheat oven to 350°F (180°C) and grease a 9-inch springform pan. For batter, whisk 1 1/2 cups flour, baking powder, baking soda, and remaining 1/4 teaspoon salt. In a large bowl, beat softened butter and granulated sugar until fluffy, about 3 minutes. Add eggs one at a time, then vanilla. With mixer on low, add flour mixture in three additions, alternating with sour cream in two, beginning and ending with flour. Mix just until smooth.

Spread half the batter in prepared pan. Sprinkle half the chilled streusel over batter. Dollop remaining batter and spread gently, then top with remaining streusel. Bake 50-55 minutes until golden and a toothpick inserted in center comes out with a few moist crumbs. If top browns too quickly, tent with foil last 15 minutes.

Cool cake in pan on a wire rack 15 minutes. Run a knife around edge, release springform, and cool another 15-20 minutes. Slice with a serrated knife.