TO PREPARE CAKE: cut a round of wax paper or parchment to fit the bottom of a 9-inch round cake pan; butter the pan, fit in the paper and then butter it.
Coat the pan with flour, tapping out the excess. Bring the cream to a boil in a heavy 3-quart saucepan; remove it from the heat and stir in both chocolates.
Cover the pan and let it stand for 5 minutes; stir the mixture until it is smooth, then set aside, uncovered.
Combine the eggs, sugar and vanilla in a large bowl; set the bowl over simmering water and whisk the mixture constantly until it is hot, about 2 minutes.
Remove the bowl from the heat and beat with an electric mixer on moderate, then high speed until the mixture has tripled in volume, cooled, and it is as thick as a soft meringue, about 8 minutes.
With a whisk, stir one-third of the egg mixture into the chocolate; gently fold in the remainder with a rubber spatula, blending until only a few chocolate streaks remain.
Pour the batter into the cake pan and smooth the top; place it in a larger pan and carefully pour in boiling water to reach halfway up the sides.
Bake in the center of the oven for 50 minutes, or until a toothpick inserted in the center comes out clean; let the cake cool for 30 minutes in the water, then remove it and place on a rack to cool completely.
When it is completely cooled, run a knife around the edge of the pan and invert onto a serving dish; gently remove the paper.
TO PREPARE THE FROSTING: Combine the chocolate and butter in the upper part of a large double boiler; set over simmering water and stir occasionally until they are melted.
Remove the pan from the heat and let the mixture cool for 10 minutes.
Add the bourbon and stir; whisk in the egg yolks, one at a time.
Beat the egg whites to form soft peaks; gradually add the sugar, beating until stiff peaks form, about 2 minutes.
Fold one-quarter of the mixture into the chocolate and blend well; fold in the remaining whites with a rubber spatula, blending until the whites are barely incorporated, then chill for at least 2 hours.
TO PREPARE THE SAUCE: Place the white chocolate in the top part of a double boiler over simmering water; stir until it is melted.
Add the cream and stir for a minute; cool to room temperature - the sauce can be chilled, then warmed slightly to blend it, then cooled to room temperature for serving.
TO ASSEMBLE THE CAKE: Scoop the frosting gently into a pastry tube fitted with a fairly wide (Number 4) star tube; pipe the mixture over the top of the cake - You can chill the cake for as long as 2 days.
TO SERVE THE CAKE: Let it reach room temperature, which will take about 3 hours.
Stir the sauce, then pour a small pool onto each serving plate; place a slice of cake on the sauce.