Preheat oven to 425 degrees and prepare a regular donut pan with non-stick spray.
In a large bowl, combine the flour, cornstarch, sugar, baking powder and salt. Mix until well combined. Using a spatula, mix in the eggs, buttermilk, butter, lemon juice and vanilla. Do not over mix.
Carefully fold in the lemon zest and fresh blueberries.
Spoon the batter into your donut pan, about ¾ of the way full. This will make about 15 donuts.
Bake for 9-12 minutes, until donuts are a light golden brown color. Once done, add the granulated sugar into a large baggie and carefully toss each donut in the sugar. It’s best to do this before allowing them to cool as the sugar will stick better.
Lay out on a rack to cool, serve warm or cooled. Store in an airtight container.