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1
Preheat oven to 350u00b0F. Line an 8 1/2-by-4 1/2-inch loaf pan, preferably metal, with parchment paper, leaving overhang on the long sides, and lightly coat with nonstick spray. Whisk flour, baking powder, and 3/4 tsp. salt in a medium bowl.
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2
Using your fingers, work grapefruit zest into granulated sugar in a large bowl until sugar starts to clump and mixture is very fragrant, about 1 minute. Add eggs, oil, and vanilla and beat with an electric mixer on high speed until light and thick, about 4 minutes. Reduce speed to low and mix in half of dry ingredients, then mix in 3/4 cup yogurt. Mix in remaining dry ingredients followed by 5 Tbsp. grapefruit juice and 1 Tbsp. poppy seeds. Scrape batter into prepared pan and smooth top.
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3
Bake cake until top is golden brown and a tester inserted into the center comes out clean, 50 to 60 minutes. Transfer pan to a wire rack. Poke holes in top of cake and brush remaining 3 Tbsp. grapefruit juice over top. Let sit 15 minutes, then run a knife around sides to loosen and use parchment paper to lift cake out of pan and onto rack. Remove parchment and let cool completely.
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4
Whisk powdered sugar, remaining 1 Tbsp. yogurt, 1 tsp. water, and a pinch of salt in a medium bowl until smooth and drizzle over cake. Sprinkle with poppy seeds and let sit until glaze is set, about 30 minutes.
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5
Loaf can be baked and glazed 3 days ahead. Store airtight at room temperature.
