1. Preheat oven to 350°F (175°C) and grease an 8 inch square pan.

2. In a bowl, cream softened butter and sugar until fluffy.

3. Beat in eggs one at a time, then stir in sour cream, vanilla extract and lemon zest.

4. In a separate bowl, whisk together flour, baking powder and baking soda.

5. Gradually fold dry ingredients into wet mixture until just combined.

6. Gently fold in blueberries dusted with a tablespoon of flour to prevent sinking.

7. Pour batter into prepared pan and smooth the top.

8. To make streusel, combine flour, brown sugar and cinnamon, then cut in cold butter until crumbly.

9. Sprinkle streusel evenly over batter.

10. Bake for 40 to 45 minutes or until a toothpick inserted in the center comes out clean.

11. Allow cake to cool before slicing and serving.