Preheat the oven to 350°F (175°C). Grease a 9-inch cake pan with butter or non-stick spray and set aside.
Combine 2 cups of flour, 1 cup of sugar, baking powder, and salt in a large mixing bowl. Mix until evenly distributed, creating a dry ingredient base.
3 large eggs
Add cold, cubed butter to the dry ingredients. Use a pastry cutter or fork to cut in the butter until the mixture resembles coarse crumbs with pea-sized bits, indicating well-incorporated fat for the streusel topping.
1 cup granulated sugar
In a separate bowl, whisk together eggs, buttermilk, and vanilla extract until smooth and slightly frothy.
3 large eggs, 1 cup buttermilk, 1 teaspoon vanilla extract
Pour the wet mixture into the dry ingredients and gently fold with a spatula until just combined, avoiding overmixing. Carefully fold in the blueberries, distributing them evenly throughout the batter.
1 cup granulated sugar
Pour the batter into the prepared cake pan, smoothing the top with a spatula. Sprinkle the streusel mixture evenly over the surface.
1 cup granulated sugar
Bake in the preheated oven for approximately 35 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean. The cake will be firm around the edges and slightly risen.
Remove from oven and allow to cool in the pan for 10 minutes. Then, transfer to a wire rack and let cool completely before serving.
