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1
Preheat oven to 400 degrees F (200 degrees C). Grease twelve 4-ounce ramekins.
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2
Stir 1/2 pound blueberries and 1/2 cup white sugar together in a saucepan over medium heat until hot and bubbly, 5 to 10 minutes. Remove from heat and cool until slightly thickened.
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3
Melt the white chocolate and butter in the top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching; remove from heat.
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4
Beat 3 tablespoons white sugar, eggs, egg yolk, and vanilla bean seeds together in a bowl with an electric mixer on high speed until light and fluffy, about 8 minutes. Add lemon juice and beat until combined.
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5
Beat white chocolate mixture and flour into egg mixture on high speed until batter reaches a thick, mousse-like consistency, 4 to 6 minutes. Pour batter into prepared ramekins to fill about 3/4 full. Spoon 2 tablespoons blueberry mixture into the center of each ramekin, poking down blueberry filling into the batter. Place ramekins on a baking sheet.
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6
Bake cakes in the preheated oven until puffed, golden, and is hot in the blueberry center, 10 to 12 minutes. Remove cakes from oven and immediately flip each onto a plate to remove cake from ramekin.
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7
Stir 1/2 pound blueberries, 1/2 cup white sugar, and water together in a saucepan; bring to a simmer and cook, stirring occasionally, until sauce is reduced by half and has a syrupy consistency, 10 to 15 minutes. Pour sauce over warm cakes and dust each with confectioners' sugar.
