Step 1: Preheat your oven to 375°F (190°C). Grease and flour a 9×13 inch baking pan, or spray with non-stick baking spray.
Step 2: Prepare the Streusel Topping: In a medium bowl, combine 1 cup all-purpose flour, 1/2 cup packed light brown sugar, 1/4 cup granulated sugar, 1 teaspoon ground cinnamon, and 1/4 teaspoon salt. Cut in 1/2 cup cold unsalted butter, diced into small pieces, using a pastry blender or your fingertips, until coarse crumbs form. Set aside.
Step 3: In a large bowl, whisk together 2 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt.
Step 4: In a separate large bowl, cream together 1/2 cup softened unsalted butter and 1 cup granulated sugar until light and fluffy, about 2-3 minutes. Beat in 2 large eggs, one at a time, mixing well after each addition. Stir in 1 teaspoon vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with 1 cup sour cream, beginning and ending with the dry ingredients. Mix until just combined. Do not overmix; a few lumps are fine.
Step 6: In a small bowl, gently toss 2 cups fresh or frozen blueberries with 1 tablespoon of flour. This helps prevent them from sinking.
Step 7: Fold the floured blueberries into the batter until evenly distributed.
Step 8: Pour the batter into your prepared baking pan and spread evenly. Sprinkle the streusel topping generously over the top of the batter.
Step 9: Bake for 40-50 minutes, or until a wooden skewer inserted into the center of the cake comes out clean. The top should be golden brown and the streusel set.
Step 10: Let the coffee cake cool in the pan on a wire rack for at least 15-20 minutes before serving. This allows the cake to set and makes it easier to cut.