1. Prepare the Donuts:
Preheat your oven to 425°F (220°C) and grease your doughnut pans with butter or non-stick spray.
In a large bowl, combine the melted butter, oil, and sugar. Whisk until smooth and fluffy, about 2 minutes.
Add the eggs, vanilla extract, lemon zest, and buttermilk to the bowl, and whisk until the mixture is smooth.
Add the flour, baking powder, baking soda, and salt. Whisk just until the ingredients are combined, then gently fold in the blueberries. The batter will be thick.
Spoon the batter into a large disposable piping bag or freezer bag (alternatively, you can spoon the batter directly into the doughnut pan). Cut off the end of the bag to create an opening about 1/2 to 3/4 inch wide—large enough for the blueberries to pass through.
Pipe the batter into the doughnut pan, filling each cavity just enough to leave space for the donuts to rise.
2. Bake the Donuts:
Bake for 7-8 minutes, or until a toothpick inserted into the donuts comes out clean.
Allow the donuts to cool in the pan for 10 minutes before transferring them to a wire rack to cool completely.
3. Make the Glaze:
In a medium bowl, combine the powdered sugar, lemon zest (if using), and 6 tablespoons of lemon juice or milk. Stir until smooth. If needed, add an additional 1-2 tablespoons of liquid to reach the desired consistency. The glaze should be thin enough to coat the donuts evenly but thick enough to set and become dry to the touch.
4. Glaze the Donuts:
Once the donuts are completely cool, dip the tops into the glaze, then overturn so the glaze runs down the sides.
Set the donuts aside for the glaze to set, or enjoy immediately.