Step 1: Preheat your oven to 350°F (175°C) and grease a 9-inch square baking pan or a round springform pan.
Step 2: In a small bowl, prepare the streusel by mixing 1/2 cup flour, 1/2 cup brown sugar, and 1 tsp cinnamon. Cut in the cold butter until the mixture is crumbly, then set aside in the fridge.
Step 3: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. Add the eggs one at a time, followed by the vanilla extract, lemon zest, and sour cream.
Step 4: Sift together the 2 cups of flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Step 5: Gently toss the fresh blueberries with a teaspoon of flour to coat them, then fold them into the batter using a spatula.
Step 6: Spread the batter evenly into the prepared pan and sprinkle the chilled streusel topping generously over the top.
Step 7: Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool for at least 15 minutes before slicing.