1/2 cup all-purpose flour
1/4 cup plus 2 tablespoons (packed) dark brown sugar
1/2 teaspoon kosher salt
1/4 cup pecans, toasted, chopped
3 tablespoons chilled unsalted butter, cut into 1/4-inch cubes
Nonstick vegetable oil spray
1 1/2 cups all-purpose flour
1 tablespoon cornmeal
1/2 teaspoon baking powder
1/2 teaspoon baking soda
1/4 teaspoon kosher salt
3/4 cup plus 3 tablespoons sugar, divided
6 tablespoons (3/4 stick) unsalted butter, room temperature
1/2 teaspoon vanilla extract
2 large eggs
1 cup buttermilk
1 tablespoon ground cinnamon
2 cups fresh (or frozen, thawed) blueberries (about 10 ounces)
1 tablespoon panko (Japanese breadcrumbs)
An 8x8x2-inch metal pan
Whisk flour, sugar, and salt in a medium bowl. Stir in nuts. Add butter; using your fingertips, work butter into dry ingredients until large, moist crumbs begin to form. Set topping aside. DO AHEAD: Can be made 5 days ahead. Cover and chill.
Preheat oven to 350°. Coat pan with nonstick spray. Line bottom with parchment paper; set aside. Whisk flour, cornmeal, baking powder, baking soda, and salt in a medium bowl; set aside.
Using an electric mixer on medium speed, beat 3/4 cup sugar and butter in a medium bowl until light and fluffy, 3–4 minutes. Beat in vanilla. Add eggs one at a time, beating to blend between additions and occasionally scraping down sides of bowl, until mixture is pale and fluffy, 3–4 minutes longer.
With mixer on low speed, add dry ingredients to bowl in 3 additions, alternating with buttermilk in 2 additions, beginning and ending with dry ingredients. Pour half of batter into prepared pan and smooth top. Whisk remaining 3 Tbsp. sugar and cinnamon in a small bowl; sprinkle evenly over batter in pan. Spoon remaining batter over; smooth top.
Toss blueberries with panko in a small bowl; scatter evenly over batter. Sprinkle crumb topping over blueberries.
Bake cake until top is golden brown and a tester inserted into the center comes out clean, 55–65 minutes. Let cool completely in pan. DO AHEAD: Can be made 1 day ahead. Store airtight at room temperature.