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Preheat the oven to 360 degrees F (180 degrees C). Grease a 9-inch round cake pan.
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Place the chocolate and butter in the top of a double boiler and allow to slowly melt.
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Beat the eggs, sugar and cornstarch in the bowl of a food processor. Process until smooth. Pour in the melted chocolate while continuing to whisk. Blend until smooth, scraping down the corners to make sure chocolate is completely mixed.
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Add Grand marnier (or preferred liquor) if using. Transfer the batter to the cake pan.
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Bake at 360 for 10 minutes then reduce oven to 250 and bake an additional 20 minutes.
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Cool in the pan on a wire rack for 10 to 15 minutes before inverting onto a serving plate. Dust with confectioners' sugar just before serving.