INGREDIENTS
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3/4 cup all-purpose flour
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1/2 cup hazelnuts, toasted and finely ground (see instructions below)
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1/4 cup unsweetened cocoa powder
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1/2 cup powdered sugar
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1/4 teaspoon kosher salt
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8 tablespoons (1 stick) unsalted butter, very cold, cut into small pieces
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1 large egg yolk
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8 ounces (1 cup) mascarpone
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4 ounces (1/2 cup) whole-milk Greek yogurt
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1/4 cup powdered sugar
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1 teaspoon grapefruit zest, finely grated
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1 teaspoon grapefruit juice
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2 large (or 3 medium) grapefruit
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1 cup sugar
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1 teaspoon light corn syrup
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pinch of fleur de sel