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1
Preheat the oven to 350F.
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2
Butter a 9-by 13-inch baking dish and line the bottom with parchment paper.
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3
Butter the paper and dust the pan with flour or cocoa powder, shaking out the excess.
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4
Set a medium heat-proof bowl over (but not touching) simmering water and add: 3 1/2 ounces unsweetened chocolate, coarsely chopped, 4 ounces semisweet chocolate, coarsely chopped, 15 tablespoons (2 sticks minus 1 tablespoon) unsalted butter.
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5
Heat until melted and smooth, stirring often.
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6
Set aside to cool.
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7
Separate: 6 eggs, at room temperature.
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8
Whisk the 6 yolks with: 1/2 cup sugar.
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9
Continue to whisk until the mixture forms a ribbon when the whisk is lifted from the bowl and all the sugar has dissolved, about 10 minutes.
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10
Fold the yolk mixture into the melted chocolate.
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11
In a separate bowl, whisk the 6 egg whites until foamy.
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12
Gradually add: 1/2 cup sugar, 1/4 teaspoon salt.
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13
Continue to whisk until the mixture looks glossy and holds a soft shape.
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14
Fold the whites into the chocolate mixture in three parts.
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15
Fold only until there are no visible streaks of white.
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16
Pour the batter into the prepared baking dish, smooth the top, and bake for 35 to 40 minutes.
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17
As the cake cooks it will develop cracks on the top; this is normal.
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18
The cake is done when the sides are set and the center is still slightly soft.
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19
Let the cake cool completely.
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20
Invert the cake onto a baking sheet, remove the parchment paper, and then invert onto a serving platter.
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21
Dust the top with: Sifted powdered sugar.
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22
Decorate the top of the cake with melted chocolate or Chocolate Sauce (page 386).
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23
Dip the tines of a fork into the chocolate and dribble it in thin lines over the top of the cake.
