Semisweet Chocolate Mousse

In a small saucepan, melt chocolate chips with water; stir until smooth. Stir a small amount of hot chocolate mixture into egg yolk; return all to the pan, stirring constantly. Cook and stir for 2 minutes or until slightly thickened. Remove from the heat; stir in vanilla. Quickly transfer to a small bowl. Stir occasionally until completely cooled. , In a small bowl, beat whipping cream until it begins to thicken. Add sugar; beat until soft peaks form. Fold into cooled chocolate mixture. Cover and refrigerate for at least 2 hours. If desired, garnish with whipped cream and raspberries.