Preheat oven to 325 degrees F.
Lightly butter a 2 quart (9 by 12 by 2 inch) oval baking dish. Melt the 1/2 pound of butter and set aside to cool.
In the bowl of an electric mixer fitted with the paddle attachment, beat the eggs and sugar on medium-high speed for 5 to 10 minutes, until very thick and light yellow. Meanwhile, sift the cocoa powder and flour together and set aside.
When the egg and sugar mixture is ready, reduce the speed to low and add the vanilla seeds, liqueur (if using) and the cocoa powder and flour mixture. Mix only until combined. With mixer still on low, slowly pour in the cooled butter and mix again just until combined.
Pour the brownie mixture into the prepared dish and place it in a larger baking pan. Add enough of the hottest tap water to the pan to come halfway up the side of the dish and bake for exactly one hour. A cake tester inserted 2 inches from the side will come out 3/4 clean. The center will appear very under-baked; this dessert is between a brownie and pudding.
Allow to cool and serve with vanilla ice cream.
**NOTE** I made mine in ramekins and baked for 40-45 minutes. Great for smaller portions.