Heat oven to 350 degrees.
In a mixing bowl, add 2 cups AP flour, 2 tsp. baking powder, and a pinch of salt. Stir together with fork.
In another bowl, add ¼ cup (½ stick) softened butter and ½ cup sugar. Whisk together on medium for 2 minutes or until light and fluffy using a KitchenAid mixer or hand mixer.
Add ¾ cup sour cream, 2 eggs, 1 tsp. vanilla extract, and the zest of 1 lemon to the bowl. Whisk for another minute or so. The mixture should be light and creamy.
In another bowl, add 1 pint of fresh blueberries and 2 Tbsp. flour. Mix together with a fork.
Add half of the liquid ingredients to the dry ingredients and mix. Repeat with the second half. Do not over-mix or the cake may become tough.
Fold the blueberries into the cake mixture. Feel free to smash some of them with a fork to give the cake color and texture.
For the streusel: Add ¼ cup (½ stick) softened butter, ⅓ packed cup brown sugar, ½ tsp. cinnamon, ⅓ cup flour, and a generous pinch of salt to a small bowl. Mix together with a fork until crumbly.
Grease an 8" springform pan (or regular round cake pan) with butter. Pour the batter into the pan and level the top. Sprinkle the streusel over the top of the cake evenly.
Place the pan in the oven and bake for 1 hour. The top should become golden as it bakes. Test the cake by poking the middle with a toothpick. The toothpick should come out clean when finished baking.
Cool slightly before serving.