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1
Start with the filling and the reduced coconut milk so that they have time to cool while the cupcakes are baking.
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2
*To make filling:* Whisk together sugar, flour, and salt in a small heavy saucepan, then add lemon juice in a slow stream, whisking until combined. Bring to a boil, whisking constantly, then simmer, whisking, until thickened, about 3 minutes. Remove from heat.
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3
Whisk yolk in a small bowl, then add about one fourth of lemon-juice mixture, whisking vigorously. Whisk into remaining lemon-juice mixture and gently boil, whisking, 1 minute. Remove from heat and stir in butter and zest. Chill until cool, at least 30 minutes.
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4
*To make reduced coconut milk:* Bring coconut milk to boil in a saucepan over medium-high heat (coconut milk will boil up high in pan). Reduce heat to medium-low; boil until reduced to 3/4 cups, stirring occasionally, 25 to 30 minutes. Remove from heat; chill (coconut milk will settle slightly as it cools).
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5
*To make frosting:* Using electric mixer, beat butter in large bowl until smooth. Add sugar, 1/3 cup reduced coconut milk, seeds from vanilla bean, coconut flakes, and salt. Beat on medium-low speed until blended, scraping down sides of bowl. Increase to medium-high and beat until light and fluffy (it will turn white or close to white as you beat it).
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6
*To make cupcakes:* Preheat oven to 325u00b0 with rack in the middle. I used two large muffin pans each with 6 muffin cups. Line with each cup with paper liner (no need to grease).
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7
Beat together yolks and 1/2 cup sugar in a large bowl with an electric mixer at high speed until pale and thick, about 3 minutes. At medium speed, beat in oil and lemon zest and juice until just combined (I used extra lemon zest because I wanted my cupcakes VERY lemony). Sift in flour and mix at low speed until just combined.
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8
Beat whites with salt in another large bowl with cleaned beaters at medium-high speed until foamy, then add remaining 1/4 cup sugar a little at a time, beating, and continue to beat until whites just hold soft peaks. Gently fold one third of whites into yolk mixture to lighten, then fold in remaining whites gently but thoroughly. Transfer batter to muffin tins, filling to top, and gently rap against counter once or twice to eliminate any air bubbles. Bake until golden brown (top may crack slightly) and a wooden pick inserted in center of cupcakes comes out clean, 30 to 40 minutes (depending on your oven). Allow to cool completely before injecting lemon filling, which can be done with a piping bag with a small tip. Just slide tip into top of cupcake and gently squeeze a small amount of filling in.
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9
Garnish with coconut flakes and Meyer lemon slices.
