Preheat oven 175 degrees C (350 degrees F). Grease one 24cm springform pan.
Heat caster sugar, butter, water, coffee and chocolate in a 3-litre saucepan over low heat, stirring constantly, until chocolate is melted and mixture is smooth; remove from heat. Beat in the eggs and pour into the prepared pan. Batter is very thin. If side and bottom of pan do not fit tightly, line the pan with foil.
Bake at 175 degrees C (350 degrees F) until a wooden pick inserted in center comes out clean, about 45 to 50 minutes. Cool completely. Remove sides of pan. Cover cake with plastic wrap and refrigerate until chilled, at least 4 hours.
Remove plastic wrap. Beat whipping cream and confectioners' sugar in a chilled 1 1/2 quart bowl until stiff. Garnish top of cake with whipped cream and, if desired, whole almonds. Refrigerate any remaining cake.