To prepare cake: Preheat oven to 350 degrees. Butter the bottom and sides of a 9- by 13-inch baking pan.
In a large bowl, sift together 2 1/2 cups flour, baking powder and baking soda.
In an electric mixer on medium-high speed, beat 1 cup butter and 1 cup granulated sugar until light and fluffy, about 2 to 3 minutes. Scrape down the bowl. Add eggs, 1 at a time, beating well after each addition and scraping the bowl down as needed. Add the sour cream, vanilla extract and salt, mixing until well-incorporated. Reduce speed to low and add the flour mixture in thirds, mixing just until everything is blended together. Gently fold in berries (the batter will be thick).
Scrape batter into prepared pan and spread it evenly with a spatula. Sprinkle streusel mixture evenly over batter. Sprinkle remaining 2 tablespoons granulated sugar over streusel. Bake until the top of the cake is golden brown and a skewer inserted into the center of the cake comes out with a few crumbs clinging but no batter, about 45 to 50 minutes, rotating the pan halfway through baking. Remove from oven and transfer pan to a wire rack to cool for at least 10 to 15 minutes. Cut cake into squares and serve warm.
Note: To toast walnuts, place the nuts in a dry skillet over medium heat, and cook, stirring (or shaking the pan frequently) until they just begin to release their fragrance, about 3 to 4 minutes (alternately, preheat oven to 325 degrees, spread the nuts on an ungreased baking sheet and bake, stirring often, for 4 to 6 minutes). Banana bread is remarkably adaptable. Add a half cup of a whole range of ingredients: chopped bittersweet or semisweet chocolate, unsweetened cocoa, shredded coconut, pitted and chopped dates, or chopped (and toasted) pecans, macadamia nuts, pistachios or unsalted peanuts. If you have unflavored yogurt, sour cream or buttermilk, add 1/4 cup after beating in the eggs, which will make the cake more moist and rich. No brown sugar? Just use 2/3 cup granulated sugar instead. If you have whole wheat flour on hand, substitute 1/2 cup of the all-purpose flour with whole wheat flour. Or follow the directions as is.