Place blueberries into a small sauce pot, heating over medium heat.
Add lemon juice, vanilla, and sugar. Stir well.
Cover (leaving a crack for the steam to escape) and cook for about 15 minutes, stirring often. (Watch it closely, it may over-run.)
Puree blueberry mixture with an immersion blender or in a stand-up blender.
Set aside.
Doughnuts:
Preheat the oven to 350 and grease a doughnut pan.
In a large mixing bowl, whisk egg, sugar, buttermilk, melted butter, blueberry syrup, and vanilla together until smooth.
Sift in all dry ingredients and whisk until all incorporated.
Fold in blueberries.
Pour half of the batter into another piping bag.
Cut the tips off both piping bags, leaving only about ½ inch opening.
Squeeze batter into the doughnut cups of the pan. Make sure there is an even amount of batter all around. (Refill the piping bag when needed.)
Bake for 10-11 minutes and let doughnuts cool on the wire rack, just until they are cooled enough to be handled.Spread paper towel under the cooling rack to catch glaze drippings.
Glaze:
Heat sauce pot over medium heat. Add whiskey and let it simmer until it's cooked down to half the amount.
Add blueberry sauce and whisk well.
Whisk in powdered sugar and heavy cream until smooth.
Dip each doughnut in the glaze and place back on the wire rack.