Heat the oven to 160 °C, or 325 °F, and butter a 9-by-12-by-2 inch oval baking dish. Melt the 1/2 pound of butter, then leave it aside until it is no longer warm.
Beat the eggs and sugar with the paddle attachment on medium-high speed for 5 to 10 minutes, and sift the cocoa powder with the flour while they mix. The egg mixture should become very thick, pale yellow, and fall from the paddle in a slow ribbon.
Lower the mixer speed and blend in the vanilla seeds, framboise liqueur if using, and the sifted cocoa-flour mixture. Stop mixing as soon as the dry streaks disappear so the whipped eggs keep their volume.
Pour in the cooled butter slowly with the mixer on low, then transfer the batter to the buttered dish. The batter should look glossy, dark, and evenly blended without losing its thick, airy body.
Set the filled dish in a larger baking pan, add hot tap water until it reaches halfway up the sides, and bake for exactly 1 hour. A tester inserted 2 inches from the edge should emerge about three-quarters clean while the center remains soft and visibly underbaked.
Let the pudding cool briefly before serving it with vanilla ice cream. The center settles slightly as it rests, but it should remain warm, loose, and fudgy beneath the crust.