1. In a small bowl, make the streusel by combining 1/2 cup of flour, 1/4 cup of granulated sugar, 1/4 cup of brown sugar, the cinnamon and 1/4 teaspoon of salt. Melt 4 tablespoons of the butter. Using a fork, stir the melted butter into the streusel ingredients until it is crumbly. Refrigerate for 10 minutes. Preheat oven to 350° F.
2. Grease an 8-inch springform pan. Line the bottom with parchment paper. In the bowl of an electric mixer, add the remaining butter, granulated sugar and brown sugar. Beat until light and fluffy. Add the eggs one at a time. Add the milk, lemon zest and vanilla. Slowly add the remaining flour, baking powder and remaining salt. Using a spatula, carefully fold in 2 cups of blueberries. Pour the batter into prepared pan. Scatter the remaining blueberries across the top. Crumble the streusel over the batter.
3. Place pan on a baking sheet, and put in the oven. Bake for 1 hour to 1 hour 30 minutes, or until top is golden and cake is cooked through. Let cake cool in the pan for 15 minutes before releasing pan. Transfer cake to a serving plate to continue cooling. Serve with whipped cream.

Per Serving: Calories: 300, Fat: 13 g (8 g Saturated Fat), Cholesterol: 60 mg, Sodium: 170 mg, Carbohydrates: 43 g, Fiber: 2 g, Protein: 4 g