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1-Preheat oven to 350°F / 180° /: Preheat oven to 350°F / 180° / gas mark 4.
2-Grease the donut pan with butter or: Grease the donut pan with butter or spray with nonstick cooking spray. Dust the pan with a light coat of flour to allow for easy removal.
3-In a large mixing bowl using an: In a large mixing bowl using an electric mixer, cream the butter and powdered sugar until light and fluffy for 2 minutes. Whisk in the buttermilk, egg, lemon juice, and lemon zest until well incorporated.
4-Add the flour and baking powder: Add the flour and baking powder. Stir by hand until the wet and dry ingredients are combined, and a thick batter is formed.
5-Transfer the dough into a pastry bag: Transfer the dough into a pastry bag equipped with a large round tip. Pipe the batter into the greased donut tin. Fill each mold 2/3 with batter.
6-Bake the donuts for 10 to 12: Bake the donuts for 10 to 12 minutes, or until a toothpick inserted near the center comes out clean. Let the donuts cool in the pan for 10 minutes. Invert the pan and release the donuts onto a wire rack.
7-While they cool, prepare the glaze: While they cool, prepare the glaze. Put the powdered sugar into a shallow, medium-sized mixing bowl. Add 2 tbsp lemon juice, whisk until a smooth glaze forms. If necessary, add more lemon juice for a thinner texture.
8-Dip the top of each donut in: Dip the top of each donut in the glaze. Flip the donuts over and put them back on the wire rack. Sprinkle each donut with lemon zest if desired. Allow the glaze to set.