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Collect the zest from one grapefruit. Cut four two-inch long strips of the zest from the second grapefruit, and slice them thinly. Collect three tablespoons of juice from either or both of the grapefruits.
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Use non stick cooking spray to coat two six-cavity donut pans.
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Preheat oven to 350 degrees.
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In a large bowl, combine flour, 1 1/2 cups sugar, baking powder, ginger, and salt, and mix well.
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In a small bowl, combine buttermilk, egg, canola oil, vanilla, and the zest of one grapefruit, and whisk until combined.
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Add the wet ingredients to the dry and stir.
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Spoon batter into pans, filling each cavity to a bit over 3/4 full.
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Bake for 25-30 minutes. Allow to cool in pan for five minutes before turning donuts out on a wire rack to cool completely.
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In a small bowl, whisk powdered sugar and grapefruit juice until smooth, then set aside.
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In a small saucepan, combine strips of zest, 3 tablespoons sugar, and 3 tablespoons water, and bring to a boil.
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Reduce heat to a low flame, and simmer until sugar has dissolved. Strain.
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Toss zest immediately in remaining unused sugar until coated.
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Transfer to cutting board and chop.
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Dip the top of each donut carefully in the glaze, then place on a wire rack glaze side up, allowing the excess to drip off. Sprinkle immediately with chopped zest.