FOR THE TOPPING
3/4 c. packed brown sugar
1/4 c. granulated sugar
2 tsp. ground cinnamon
1/4 tsp. kosher salt
3/4 c. (1 1/2 sticks) butter, melted
1 1/2 c. all-purpose flour
FOR THE CAKE
3/4 c. (1 1/2 sticks) butter, softened, plus more for greasing
2 1/2 c. plus 2 tbsp all-purpose flour, divided
1 tsp. baking soda
3/4 tsp. baking powder
1/2 tsp. kosher salt
1 1/2 c. granulated sugar
2 large eggs
1 1/3 c. sour cream
2 tsp. pure vanilla extract
2 c. blueberries
Step 1
Make the topping: Preheat oven to 350°. In a medium bowl, whisk together sugars, cinnamon, and salt. Add melted butter and stir to combine. Add flour and stir until moist clumps form.
Step 2
Make the cake: Butter a 9"-x"-13" baking pan and line with parchment, leaving a 2-inch overhang. Butter parchment. In a medium bowl, whisk 2 1/2 cups flour, baking soda, baking powder, and salt.
Step 3
In a large bowl, beat room-temperature butter and sugar until light and fluffy. Add eggs one at a time, beating until each is well combined. Add flour mixture, sour cream, and vanilla and beat until just combined.
Step 4
In a medium bowl, toss blueberries with remaining 2 tablespoons flour and add to batter. Fold gently, just until blueberries are combined. Scrape cake batter into prepared baking dish and spread top evenly with a spatula. Sprinkle prepared topping evenly over batter.
Step 5
Bake until a toothpick inserted into the center comes out clean, 50 to 60 minutes. Let cool completely.
Step 6
Cut cake into squares and serve slightly warm or at room temperature.