In a heavy 3- to 4-quart pan over low heat, stir sugar with water until sugar is dissolved and mixture looks clear. Increase heat to high and boil mixture, without stirring, until dark golden brown, about 10 minutes; occasionally brush sugar crystals down sides of pan with a wet pastry brush, and when mixture begins to caramelize around edge of pan, lift pan and gently swirl to ensure even caramelization. Remove from heat and carefully add 3/4 cup whipping cream (mixture will bubble up); stir gently until smooth. Let cool for 5 minutes, then stir in finely chopped bittersweet chocolate until well blended. Transfer mixture to a large bowl and let cool to room temperature, about 1 hour.
Stir Frangelico or other hazelnut-flavored liqueur into chocolate mixture. In another bowl, with a mixer on high speed, beat 1 cup whipping cream to firm peaks. Gently fold about a third of the cream into the chocolate mixture, then fold in remaining cream until no white streaks remain.
Spoon mousse into dessert glasses and chill until cold, at least 1 hour, or up to 1 day. Garnish with berries or chocolate shavings if desired.