Preheat oven to 350F degrees, spray 9x13 baking dish with nonstick cooking spray, in a medium bowl combine biscuit mix and sugar, add sour cream, eggs, milk and vanilla extract, mix until combined, gently stir in blueberries by hand, scoop batter into prepared baking dish and make sure it is evenly spread, in a bowl, combine sugar, flour and butter, using your fingers or a pastry cutter, start combining mixture until it looks coarse and crumbly, sprinkle mixture over dough, bake about 30-35 minutes until a toothpick inserted comes out clean
Preheat oven to 350F degrees, spray 9x13 baking dish with nonstick cooking spray, in a medium bowl combine biscuit mix and sugar, add sour cream, eggs, milk and vanilla extract, mix until combined, gently stir in blueberries by hand, scoop batter into prepared baking dish and make sure it is evenly spread, in a bowl, combine sugar, flour and butter, using your fingers or a pastry cutter, start combining mixture until it looks coarse and crumbly, sprinkle mixture over dough, bake about 30-35 minutes until a toothpick inserted comes out clean
Folding fresh blueberries into a heavy, biscuit-mix-based batter requires deliberate gentleness to prevent the fruit from bursting and bleeding purple streaks throughout the crumb. Because the dough base is exceptionally dense, incorporating the berries demands manual patience rather than aggressive stirring with an electric mixer. Cold butter cut into small cubes must be worked into the flour and sugar mixture using fingertips or a pastry cutter until the texture resembles coarse, uneven gravel, which ensures the topping bakes into a distinct, crumbly crust rather than melting into a solid sheet of sugar. Setting the oven to 350 degrees Fahrenheit provides the moderate heat necessary to cook the thick center without burning the exterior crumbs. Testing for doneness at the thirty-minute mark with a wooden toothpick inserted into the thickest portion of the cake prevents over-baking, as even two extra minutes in a dry heat environment will dehydrate the sour cream crumb. A 9x13-inch baking dish coated thoroughly with nonstick spray ensures easy release, though bakers lacking this specific pan size can substitute an appropriately sized roasting pan or two smaller square pans while adjusting baking times downward to account for different depth profiles.