In a small bowl, whisk together the cornstarch and water until smooth. Set aside.

In a medium saucepan over medium heat, 2 cups blueberries (set aside ½ cup), granulated sugar, and lemon juice. Cook, stirring frequently, until the blueberries begin to burst and release their juices, about 4–5 minutes.

Whisk in the cornstarch slurry and continue cooking for 1–2 minutes, stirring constantly, until the mixture thickens into a glossy, jam-like consistency.

Remove from heat and stir in the vanilla extract. Set aside to cool slightly; the mixture will continue to thicken as it sits.

Preheat the oven to 350°F (180°C). Grease a 9-inch springform pan and line the bottom with parchment paper.

In a large mixing bowl, beat the softened butter and granulated sugar together until light and fluffy, about 2–3 minutes.

Add the eggs, vanilla extract, and sour cream. Mix until smooth.

In a separate bowl, whisk together the flour, baking powder, and salt.

Add the dry ingredients to the wet ingredients in two additions, alternating with the milk. Mix just until combined, do not overmix.

Spread the batter evenly into the prepared pan.

In a bowl, whisk together the flour, brown sugar, cinnamon, and salt.

Pour in the melted butter and stir until the mixture forms a soft, sandy streusel.

Spoon the blueberry mixture evenly over the cake batter, gently spreading it to the edges. Distribute the remaining ½ cup fresh blueberries over top. Sprinkle the streusel evenly over the blueberry layer.

Bake for 45–50 minutes, or until the streusel is golden and a toothpick inserted into the cake layer comes out clean. (without batter, blueberry is okay)

Allow the cake to cool in the pan for 20 minutes, then remove the springform ring and let cool completely before slicing.

In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth and pourable.

Drizzle the glaze lightly over the cooled cake using a spoon or piping bag. Slice and enjoy!