In the top of a double boiler over barely simmering water, melt the unsweetened and semisweet chocolates together. The chocolate should be just warm.

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.

In the bowl of an electric mixer, using the whisk attachment, bear the yolks at high speed until thick and lemon colored. While the yolks are beating, combine the sugar and water in a small pot and bring to a boil over high heat. Boil exactly 90 seconds without stirring. Slowly pour the sugar mixture into the egg yolks, continuing to beat. Add the optional cognac.

Replace the whisk with the paddle attachment, reduce the speed to medium-low, and slowly add the chocolate mixture. 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. Add the butter piece by piece and mix well.

Pour the chocolate mixture into a large bowl and whisk in half the whipped cream. With a spatula, gently fold in the remaining whipped cream.

Pour the mousse into 8 to 10 serving glasses or bowls and refrigerate at least 3 hours, or up to 3 days.