In a small saucepan, melt the butter over moderately low heat, stir in the espresso, the sugar, the zest, the orange extract and the grand marnier, and transfer the mixture to a bowl.
In the top of a double-boiler set over barely simmering water, melt the semisweet and the unsweetened chocolate, stirring occasionally.
Stir the chocolates into the butter mixture, add the eggs, stirring until the mixture is just combined.
Line the bottom of a buttered 9" cake pan, 2" deep, with parchment paper and butter the paper; spoon the mixture into the pan, put the pan in a baking dish, and add enough hot water to the dish to reach halfway up the side of the pan.
Bake the cake in the middle of preheated 350 degree oven for 1 hour.
Remove the cake from the water bath, let it cool on a rack. Chill it, covered loosely, for 3 hours.
Run a thin knife around the inside of the pan and dip the bottom of the pan in hot water for 3 seconds.
Invert a platter over the pan, invert the cake with a sharp rap onto the platter, and cut it with a sharp knife dipped in hot water, cleaning the knife before cutting each slice.