In a saucepan over medium heat, combine fresh blueberries, water, and granulated sugar. Bring to a low boil, mix in cornstarch slurry to thicken, simmer for about 10 minutes until glossy, then stir in vanilla and set aside to cool.

Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan. In a bowl, whisk together flour, baking powder, baking soda, sugar, and salt. Add softened butter and beat until resembling coarse crumbs, then mix in eggs, buttermilk, lemon juice, zest, and vanilla until smooth. Fold in cooled blueberry filling.

In another bowl, mix softened butter, flour, brown sugar, lemon zest, and cinnamon until crumbly to create the filling.

Pour half of the batter into the pan, spread evenly. Sprinkle cinnamon filling on top, dollop remaining batter and spread. Finally, pour cooled blueberry filling evenly.

In a small bowl, combine melted butter, flour, sugar, cinnamon, and lemon zest until crumbly for the streusel topping.

Bake for 50-60 minutes until a toothpick comes out clean or with moist crumbs. Let cool in pan for at least 15 minutes.

Make the glaze by mixing powdered sugar, milk, and vanilla in a bowl until smooth. Adjust milk for desired consistency.

Once the cake is cooled, slice into squares and drizzle with glaze.