Preheat the oven to 300°F.

Cut a piece of parchment to fit the bottom of the cake pan, spray the pan with nonstick spray, and line it with the paper. Press the paper flat and spray the top surface.

Put 1 cup of the sugar in a saucepan with ¼ cup water and bring to a boil over medium heat, stirring to dissolve the sugar. Turn off the heat.

Heat water in the bottom of a double boiler over medium heat, put all the chocolate in the top, and melt it over hot water, stirring so it melts evenly. Make sure all the chocolate is melted. Remove the chocolate from the heat and, stirring constantly with a rubber spatula, add the butter bit by bit until is has melted smoothly into the chocolate. Make sure all the butter has melted, then add the sugar syrup and stir it in well. When all is well combined, stir in the Grand Marnier.

Beat the eggs with the remaining ¼ cup sugar until they are thick and foamy. This will take 5 or 6 minutes in an electric mixer, a little longer by hand. Add about 1/3 of the beaten eggs to the chocolate mixture and, using a rubber spatula, fold them in gently but thoroughly; then add the chocolate mixture to the remaining eggs and fold the two together. Pour the batter into the prepared cake pan.

Put the cake pan into a larger roasting pan and add water to come a third of the way up the side of the cake pan. Bake for 1 hour. The cake is done when it feels springy to the touch and is just beginning to come away from the sides of the pan. Turn the cake out onto a cake plate and peel off the parchment. Serve warm with English Custard Sauce.