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1
In the top of a double boiler over barely simmering water, melt the unsweetened and semisweet chocolates together.
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2
The chocolate should be just warm.
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3
In the bowl of an electric mixer, using the whisk attachment (or with a hand-held mixer), whip the cream at high speed until it forms soft peaks: chill.
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4
In the bowl of an electric mixer, using the whisk attachment, bear the yolks at high speed until thick and lemon colored.
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5
While the yolks are beating, combine the sugar and water in a small pot and bring to a boil over high heat.
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6
Boil exactly 90 seconds without stirring.
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7
Slowly pour the sugar mixture into the egg yolks, continuing to beat.
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8
Add the optional cognac.
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9
Replace the whisk with the paddle attachment, reduce the speed to medium-low, and slowly add the chocolate mixture.
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10
Beat 4 to 5 minutes, scraping the bottom of the bowl once or twice, until both the mixture and the bowl feel warm but not hot.
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11
Add the butter piece by piece and mix well.
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12
Pour the chocolate mixture into a large bowl and whisk in half the whipped cream.
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13
With a spatula, gently fold in the remaining whipped cream.
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14
Pour the mousse into 8 to 10 serving glasses or bowls and refrigerate at least 3 hours, or up to 3 days.
