Preheat the oven to 350°F (175°C). Grease a 9-inch round or square baking pan with butter or non-stick spray. In a mixing bowl, combine the flour, baking powder, salt, and half of the sugar; whisk until well blended.
Cut the cold butter into small cubes and add it to the flour mixture. Using a pastry cutter or your fingers, cut in the butter until the mixture resembles coarse crumbs with some pea-sized pieces. Set aside the streusel topping.
In a separate bowl, whisk together eggs, buttermilk, vanilla extract, and remaining sugar until smooth and slightly frothy. Pour the wet ingredients into the dry ingredients and gently fold until just combined; avoid overmixing, as this can make the cake dense.
Gently fold the blueberries into the batter, being careful not to crush them. Pour the batter into the prepared baking pan and spread evenly with a spatula. Sprinkle the reserved streusel topping over the surface.
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 should start to pull away slightly from the sides of the pan. Let it cool for 10 minutes before serving.
Slice and serve the coffee cake warm or at room temperature, showing a tender crumb dotted with blueberries and a crisp streusel topping.