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Preheat oven to 350. Grease muffin tin or line with muffin liners and set aside.
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In a mixing bowl, beat the egg whites until soft peaks form. Transfer to a small bowl and put in the fridge.
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In the bowl of a stand mixer, cream butter and sugar until light and fluffy, about 3 minutes. Add the egg yolks, one at a time, until well-combined.
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Pour the flour, baking powder, baking soda, poppy seeds, zest, and salt on top of the butter/sugar/egg mixture and beat for about 5 seconds. Add the buttermilk and beat again for just a few seconds. Add the grapefruit juice and vanilla. Beat just until smooth.
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With a rubber spatula, fold the egg whites into the batter until just blended.
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Spoon into muffin cups and then sprinkle with sugar.
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Bake until golden brown and springy to the touch, 20-25 minutes.
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Let cool slightly before removing and serving with grapefruit cream.