Preheat oven to 350°.  Butter or spray a loaf pan or an 8×8 pan. 

To make the coffee cake:  Whisk all dry ingredients in a medium-size bowl. Add in the egg, sour cream, butter, and vanilla, mixing until well incorporated. Fold blueberries into the batter gently with a spatula. The batter will be very thick and almost dough-like. Spread into prepared pan. 

To make the streusel:  In a small bowl, mix ingredients for streusel until they are like coarse crumbs. Sprinkle on top of unbaked coffee cake. 

Bake for 35-40 minutes in the 8×8, or 45-55 minutes in the loaf pan {until a toothpick inserted in the center comes out clean and the top is a light golden brown}. Let cool, cut into slices or squares, serve warm or cool. 

To make the cream cheese topping:  Combine all ingredients in a small bowl, and whisk well or mix with a hand mixer or standing mixer until smooth. Spoon onto coffee cake right before serving and top with additional berries if desired.  Store in an airtight container at room temperature for up to three days. Enjoy!