This last weekend we attended a potluck for our daughters' swim team. I brought this sour cream blueberry coffee cake because I didn't have any buttermilk for my original recipe. As a result, the sour cream version is heavenly. But what I love the most is the streusel topping. The streusel topping is versatile for any cake or muffin recipe calling for streusel. Full of brown sugar and cinnamon, the texture of this streusel topping is perfect. Enjoy!