Preheat the oven to 350°F and line an 8″x 8″ pan with parchment paper.
Prepare the streusel. In a medium mixing bowl, stir together the flour, granulated sugar, brown sugar, and salt. Add in the butter using a pastry blender or your hands and mix together until coarse crumbs form. Place the bowl in the refrigerator while preparing the cake batter.
In a medium mixing bowl, stir together the flour, baking powder, and salt. Set aside.
In a large mixing bowl, stir together the oil, sugar, lemon juice, lemon zest, lemon extract (if using), vanilla extract and eggs. Once well combined, gently fold in the flour mixture. Do not over mix. Carefully fold in the blueberries.
Pour into the prepared pan and sprinkle the streusel evenly over the top.
Bake for 55-65 minutes or until a toothpick inserted in the center of the cake comes out clean. A few moist crumbs are okay but there shouldn’t be wet batter on the toothpick.
Let cool for 15 minutes and then serve. Cover and store leftovers at room temperature for up to 3 days.
