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Arrange a large bamboo steamer or a large vegetable steamer over simmering water.
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Make sure it is large enough to hold the baking pan.
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Line a 9 inch square pan with waxed paper.
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Separate the eggs.
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Place the yolks in a large bowl along with the sugar and water.
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Beat with an electric mixer on medium speed until the mixture has increased about three times in volume.
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Whisk together the flour and the baking powder; sift this over the egg mixture, and fold in gently.
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Blend in the extract.
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Whip the egg whites in a clean bowl to stiff, not dry, peaks.
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Fold into yolk base.
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Pour the batter into the prepared pan, and smooth out to edges.
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Rap the pan on the counter to get rid of large air bubbles.
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Place the pan into the steamer.
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Cover the top of the steamer with a kitchen towel, and place the lid on top; the towel will absorb any steam that collects on top from dripping onto the cake.
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Steam for about 20 minutes, or until a toothpick inserted into the center comes out clean.
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Cool on a wire rack.
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Dust with confectioner's sugar.