Step 1: In a medium bowl, whisk together 1/2 cup all-purpose flour, 1/4 cup granulated sugar, 1/4 cup packed light brown sugar, and 1 teaspoon ground cinnamon.
Step 2: Add 1/4 cup cold unsalted butter, cut into small cubes. Using your fingertips, a pastry blender, or two knives, cut the butter into the flour mixture until coarse crumbs form. Refrigerate while preparing the cake batter.
Step 3: Preheat your oven to 375°F (190°C). Grease and flour a 9-inch square baking pan or a 9-inch round springform pan. Line the bottom with parchment paper if using a springform.
Step 4: In a large bowl, whisk together 1 3/4 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt. Set aside.
Step 5: In a separate large bowl, using an electric mixer, cream together 1/2 cup unsalted butter (room temperature) and 1 cup granulated sugar until light and fluffy, about 2-3 minutes.
Step 6: Beat in 2 large eggs, one at a time, mixing well after each addition. Add 1 teaspoon vanilla extract and 1/2 cup sour cream, mixing until just combined.
Step 7: Gradually add the dry ingredients (from Step 4) to the wet ingredients (from Step 6), mixing on low speed until just combined. Do not overmix; a few lumps are okay.
Step 8: In a small bowl, gently toss 1 1/2 cups fresh or frozen blueberries with 1 tablespoon of all-purpose flour. Gently fold the floured blueberries into the cake batter until evenly distributed.
Step 9: Pour the batter into your prepared baking pan and spread evenly. Sprinkle the chilled streusel topping generously over the entire surface of the batter.
Step 10: Bake for 40-50 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean (a few moist crumbs are fine). If the streusel starts to brown too quickly, loosely tent the cake with aluminum foil.
Step 11: Let the coffee cake cool in the pan on a wire rack for at least 15-20 minutes before serving. For best results, allow it to cool almost completely before slicing.