Position rack in middle of oven. Heat oven to 325 degrees F. Butter the bottom of an 8-inch round pan; line the bottom of the pan with a parchment-paper or waxed-paper round; butter the paper.
Combine the sugar and water in a small saucepan. Bring to boiling over low heat, stirring occasionally to make sure all the sugar crystals dissolve. Remove the saucepan from the heat. Stir in the butter and chocolate pieces; let stand for 5 minutes. Then whisk the mixture until smooth.
Whisk liqueur into chocolate mixture. Then whisk in eggs, one at a time; be careful not to overmix.
Pour the batter into the prepared round pan. Place the pan in a small roasting pan. Place on the oven shelf. Pour 1 inch of warm water into the roasting pan.
Bake in 325 degree F oven 45 minutes or until mousse cake is set and slightly dry on the surface. Remove cake pan from water bath in roasting pan. Place cake pan on wire rack and let cool to room temperature. Cover with plastic wrap. Refrigerate cake in pan until well chilled.
To unmold, run a thin knife between the cake and the pan. Pass the bottom of the pan over heat for no more than 10 seconds. Invert the cake onto a serving platter.
Beat together the heavy cream and sugar in a small bowl until soft peaks form. Spread the whipped cream over the top of the cake. Decorate the top with fresh raspberries, if desired. Makes one 8-inch cake.