Preheat your oven to 375°F and prepare a 9x9-inch baking pan with parchment paper or light greasing. This ensures easy removal and prevents sticking. Having everything ready before you begin makes the process smoother and more enjoyable.

Whisk together flour, sugar, baking powder, and salt in a large mixing bowl. Make a well in the center, then add oil, eggs, milk, and vanilla extract. Stir until just combined—do not overmix, as this develops gluten and toughens the cake. The batter should look slightly lumpy.

Gently fold fresh blueberries into the batter using a spatula, being careful not to crush them. Reserve a few berries for the top if desired. Pour the batter into your prepared pan and spread evenly. This cozy blueberry coffee cake summer recipe benefits from even distribution for consistent baking.

Prepare the streusel by combining flour, sugar, and cinnamon in a small bowl. Cut cold butter into small cubes and work into the dry mixture using your fingertips until resembling coarse breadcrumbs. This texture creates the ideal topping consistency. Don't overmix or the topping becomes dense.

Sprinkle the streusel mixture evenly over the batter, covering as much surface as possible. Bake for 38-42 minutes until a toothpick inserted in the center comes out clean with just a few moist crumbs. The top should be golden brown and slightly firm to the touch.

Remove from the oven and let cool in the pan for 10 minutes before turning out onto a wire rack. Cooling slightly in the pan allows the cake structure to set properly. Complete cooling takes about 30 minutes total, but it's delicious served warm with coffee.