Put water into bottom part of a double boiler. Bring to a simmer and be sure the water does not touch the bottom of the top part. Let the top part heat.
Put cream, sugar, and salt into a saucepan. Stir-cook until sugar dissolves and cream is hot. Turn off heat.
Put chocolate (both) and Cognac into a small enameled pan. Bring to a boil, remove, cover, and set into a pilot heated oven, or into very hot water. Let chocolate soften. Do not stir.
Take butter from the refrigerator. Cut it into pats. Put butter into a mixer bowl and beat until it is fluffy and white. This will take about 15 minutes. The butter must be cold when it is beaten, or it tends to melt and not whip.
Put eggs into a bowl and beat with a whisk. Mix well, but do not whip to a froth. Gradually pour in the hot cream and continue to mix. Pour this into the hot double boiler. Stir-cook with a wooden spoon until mixture thickens to the consistency of whipping cream. Stir-cool in cold water, and then in ice water. Strain sauce into a clean pan. Add the vanilla.
When the butter is whipped and the cream sauce is cold, combine the recipe. Stir chocolate and Cognac together. Cool slightly, quickly blend half of the chocolate into the butter, and add the rest before the chocolate hardens. Mix well. Scrape down sides of the bowl. Very slowly pour the very cold custard into the chocolate-butter mixture while beating. The mixture will become creamy and smooth in about 5 minutes of constant beating.
Butter an oblong mold (I use a 2-cup pâté mold) and sprinkle with crumbs. Spoon mixture into mold. Smooth the top, cover with more crumbs, cover with transparent wrap and refrigerate.
To serve, dip the mold into warm water, dry it off and then invert the mold onto a serving plate. Cut into slices and serve with crisp cookies. Cigarette cookies are perfect with this dessert. Make them in advance, freeze, and serve them frozen.