Preheat oven to 375°F (190°C). Grease an 8-inch baking pan.

In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla extract.

In a separate medium bowl, whisk together the flour, baking powder, salt, and cinnamon.

Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.

Gently fold in the blueberries.

Spread the batter evenly into the prepared baking pan.

For the streusel topping, in a small bowl, combine the flour, granulated sugar, brown sugar, and cinnamon. Cut in the hardened butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs.

Sprinkle the streusel topping evenly over the batter.

Bake for 30-35 minutes, or until a toothpick inserted into the center of the cake comes out clean. Let cool in the pan before slicing and serving.