Preheat the oven to 180C/350F/Gas 4. Grease a doughnut pan, and set aside.
Whisk together the flour, sugar, baking powder, baking soda, and salt. Set aside.
Whisk together the egg, buttermilk, oil, vanilla, lemon juice, and zest until combined.
Add to the dry ingredients, and mix until just combined. Gently fold in the blueberries. Do not overmix.
Spoon the batter into the prepared doughnut pan and smooth out the tops.
Bake for 10 - 12 minutes or until a toothpick inserted comes out clean. Allow to cool for 5 - 10 minutes before transferring them to a wire rack to cool completely.
For the Glaze
Whisk together the icing sugar, lemon juice, vanilla, and milk until smooth and combined.
Dip each doughnut into the glaze, and leave to set on the wire rack.
Video
Notes
*To make your own buttermilk, simply add 1/2 tablespoon of fresh lemon juice or white vinegar to 1/2 cup (120ml) of milk. Stir to combine, and allow to sit for 5 - 10 minutes until slightly thickened and curdled.Leftover doughnuts can be stored in an airtight container at room temperature or in the fridge for up to 2 days. They also freeze well for up to 2 months. Thaw overnight in the fridge before serving.