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Preheat oven to 350 degrees F
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Grease an 8-inch round cake pan with butter and dust lightly with flour. Line the bottom with parchment paper.
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Chop the chocolate into small pieces and put in the top of a double boiler with the butter over medium-high heat. Melt the chocolate and butter, stirring occasionally. Add the sugar and stir well. Remove from heat and let cool for 10 minutes.
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In a medium bowl, whisk the eggs until frothy and blended. Add to the chocolate and butter and whisk until thick and smooth. Add the flour and whisk again.
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Pour into prepared cake pan and bake until the top is set and no longer appears wet, about 40 minutes. Resist the urge to open the oven during cooking.
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Remove from oven and cool completely.
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Run a knife around the edge of the cake, set a plate over the pan, and invert the cake onto the plate. Then, set another plate on top of the cake and flip the cake onto the plate. Its top should be upright.
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Sprinkle the lightly with confectioners' sugar.
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Serve in thin slices. If desired, top with whipped cream and/or berries.
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Store leftovers in the refrigerator.