Butter and flour a 30 cm springform pan*. Break the chocolate into chunks and put in the top of a double boiler along with the butter. Melt over simmering water, stirring occasionally. Stir in 1.25 cups sugar into the melted chocolate and butter mixture and continue heating until the sugar is dissolved. Beat the egg yolks in a separate bowl. Beat some of the hot chocolate mixture into the yolks, then return all the mixture to the sauce pan. Cook over simmering water, stirring constantly, until slightly thickened; then stir in the Grand Marnier and vanilla. Beat the egg whites until they stand in soft peaks when the beater or whisk is removed. Continue beating, gradually adding the remaining quarter cup sugar until the whites stand upright in stiff peaks when beater is removed. Fold egg whites and chocolate mixture together carefully. Pour the mixture into the prepared pan. Bake for 3 hours at 250. Remove from the oven. Let cool at room temperature, cover and chill. This cake is great served with a creme anglaise. *For Passover you can use matzoh meal in place of flour. ----