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Preheat oven to 400 degrees F and grease your doughnut pans with butter or non-stick spray.
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In a large bowl, add the butter, oil and sugar. Whisk for a couple minutes until smooth and fluffy.
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Add in the egg, vanilla, lemon zest and milk. Whisk until smooth.
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Add the flour, baking powder, and salt. Stir until combined (don't over stir). The batter will be very thick.
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Gently fold in blueberries.
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Add batter to a large disposable freezer bag (or piping bag). Cut off the corner of the bag, just big enough for the blueberries to get through, then pipe the batter into each donut hole. You want a smooth, even piping of the dough if you want smooth, evenly shaped donuts. The donut hole should be almost filled completely but do not over fill.
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Bake for 8-9 minutes. I like my edges to just start to brown a little and a toothpick inserted comes out clean done.
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Let cool 10 minutes before removing from the pan to cool completely.
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When cooled and you are ready to enjoy, make the glaze.
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In a medium bowl, combine sugar, zest, and lemon juice.
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When donuts are cooled, either brush the glaze on top of the donuts or dip the tops of the donuts into the glaze. Set right side up and allow the glaze to set.