Preheat the oven to 425°F. Grease two donut pans with butter or spray with nonstick spray.
In a large bowl, combine the sugar, butter, and oil and whisk about 2 minutes, until smooth.
Add the buttermilk, eggs, vanilla, and lemon zest and whisk until smooth.
Add the flour, baking powder, salt, and baking soda, and whisk until just combined. Stir in the blueberries. (The batter will be thick.)
Spoon the batter into a large disposable piping bag or zip-top bag. Cut off the end of the bag so the opening is about ½- ¾” wide. (Big enough for the blueberries to get through!) Pipe the batter around each donut hole. (The batter should just fill the indent because the donuts will expand and rise.)
Bake for 7 to 8 minutes, until a toothpick inserted comes out clean. Let cool for 10 minutes before transferring the donuts from the pan to a wire rack to cool completely.
Make the Glaze
In a medium-sized shallow bowl, combine powdered sugar and 6 tablespoons of the juice. Add an additional 1 to 2 tablespoons of juice if needed. (You want the glaze to be thin enough to evenly coat your donuts, but thick enough that it will set completely.)
When the donuts are completely cool, dip the tops into the glaze, overturn and set on a cooling rack so the glaze runs down the sides. Set aside for the glaze to set. (It happens quickly!)
Video
Notes
*The batter will be thick – that’s okay! I find the easiest way to get it into the pan is using a piping bag, but you can scoop with a spoon if you don’t have one. *If you like more lemon flavour, use fresh lemon juice for the glaze. If you like less lemon flavour, use milk. *If you’re making these ahead of time, I recommend baking them, letting them cool, and storing in an airtight container, then glazing up to a few hours before serving them. The glazed doughnuts become softer and stickier when stored in an airtight container. I baked and glazed them in the morning, and let them sit on the counter the whole day so the glaze would stay set.