Preheat oven to 350°F. Spray your donut pan with non-stick cooking spray. Set aside.
Measure out ½ cup of blueberries, reserving the rest for another use. Drain berries (reserving the juice) and gently pat dry with a paper towel. Toss with cornstarch until evenly coated. Set aside.
In a large bowl, whisk together the flour, baking powder, and salt. Set aside.
In a separate bowl, mix together the milk, sugar, egg, butter, lemon juice, lemon zest, and vanilla, until fully combined. Add the dry ingredients to the wet mixture and stir until just combined, making sure not to overmix. Gently fold the cornstarch coated blueberries into the batter.
Evenly distribute the batter between 6 of the prepared donut pan holes. Bake
Bake at 350°F for 9-10 minutes, or until a toothpick inserted into the donut comes out clean. Remove from the oven and allow to cool for a few minutes, then invert onto a wire rack to finish cooling.
To make the blueberry glaze:
In a small bowl, whisk together the powdered sugar and 2 tablespoons of the reserved Blueberry syrup until well combined. Whisk in lemon zest. Add more blueberry juice to thin out the glaze or add more powdered sugar to thicken the glaze as needed.
Dip each donut into the glaze and place back onto the wire rack to allow the glaze to dry and harden. Once the glaze is set, they are ready to eat!