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For the Cake: Preheat the oven to 375°F/190°C. Coat the inside of an 8-inch by 2-inch deep (20 cm x 5 cm) heart shaped cake pan with nonstick spray, line bottom with parchment, then spray parchment. (Cut the parchment to fit by tracing the pan).
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Place the whole eggs, in their shells, in a bowl filled with hot tap water for 5 minutes. Warming the eggs ensures maximum volume when they are whipped.
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Place the chocolate and butter in a large heatproof bowl and set over a pot of simmering water or microwave until chocolate is three-quarters of the way melted. Remove from heat source and whisk gently every now and then for a few minutes until chocolate is completely melted and mixture is smooth.
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Meanwhile, crack whole eggs into bowl and beat with mixer on high speed until tripled in volume, pale yellow and thick enough to hold a very soft peak.
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Add about 1/4 of the egg mixture to the cooled chocolate. Gently combine by hand, using a whisk. It’s OK if streaks of egg remain. Add remaining eggs and fold in, first using the whisk, then finishing with a large silicone spatula. The batter will deflate a bit, but try to retain as much volume as possible. The mixture will look like chocolate mousse. Scrape batter into pan and level with a small offset spatula.
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Place the pan in a large roasting pan filled with 1-inch of hot water. Bake for about 12 to 15 minutes. The surface will look dull. If you tilt the pan slightly, the edges will come away from the sides of the pan. Both of those visuals are important and the only way to really tell that it is done. It will still be very soft, like a pudding. Don’t fret. Remove pan from water and cool pan completely on rack. Wrap with plastic wrap and refrigerate overnight. (Cake may be frozen at this point up to 1 week; defrost in refrigerator overnight before continuing).
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For Unmolding: Unwrap pan, flip it over (the cake will not fall out) and allow hot tap water to run all over the bottom of the pan. The warmth from the water should loosen the cake from the sides and bottom of the pan. Warm an icing spatula under hot water and blot dry. Run the spatula around the sides of the cake. Apply pressure out towards the pan, not in towards the cake, or you might accidentally shave off some of the cake’s side. Flip the cake over and firmly shake and wiggle the pan back and forth; you are trying to get the cake to release its surface tension with the pan. The cake should slip out. If it doesn’t, repeat the warm water and jiggling steps. Gently lift off the pan completely. Peel off the parchment.
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For Glaze & Decoration: Place cake on rack set on a sheet pan. Pour the ganache glaze over the cake and use an offset spatula to gently spread the cover the top of the cake and allow to drip down the sides. Use spatula to help the ganache adhere to the sides as well. Allow the cake to sit for several minutes for the glaze to begin to set.
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Use a strong broad spatula to lift cake and place on serving platter. Cake may be refrigerated at this point up to overnight.
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Right before serving, arrange fresh berries on top of cake as desired. I made a single row around the outer edge, but you might create a different design. Feel free to use the entire half-pint. Cake is ready to serve cold or at room temperature.