Preheat the oven to 350°F (175°C). Grease the baking pan with butter or non-stick spray, then set aside.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt until evenly combined. Measure and melt the butter separately.
2 cups all-purpose flour
Add the beaten eggs, milk, and melted butter to the dry ingredients. Stir gently until just combined, avoiding overmixing. Fold in the fresh blueberries carefully into the batter.
2 cups all-purpose flour
Pour the batter evenly into the prepared baking pan, smoothing the top with a spatula. In a smaller bowl, prepare the streusel topping by combining brown sugar, flour, and cold cubed butter. Use your fingers or a pastry cutter to blend until crumbly and peek through the mixture should resemble coarse crumbs.
2 cups all-purpose flour
Sprinkle the streusel evenly over the batter. Bake in the preheated oven for 35-40 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean. The edges should be slightly pulling away from the sides of the pan, and the aroma of baked blueberries and streusel should be fragrant.
2 cups all-purpose flour
Allow the cake to cool in the pan for about 10 minutes, then transfer to a wire rack if desired. Serve warm or at room temperature, optionally with a dusting of powdered sugar or a dollop of whipped cream for extra flair.