PREPARATION:

1. Heat 1 cup chocolate chips in medium saucepan over low heat until melted, stirring frequently. Remove from heat and stir in 1/4 cup whipping cream.
2. Place egg yolks in medium bowl. Whisk about half of chocolate mixture into egg yolks; whisk egg yolk mixture back into chocolate mixture in saucepan. Cook over low heat 2 minutes, whisking constantly. Remove from heat; cool 3 to 5 minutes.
3. Beat remaining 1-3/4 cups whipping cream and vanilla to soft peaks in medium bowl. Gradually beat in sugar; continue beating until stiff peaks form. Stir about one-fourth of whipped cream into chocolate mixture; fold chocolate mixture into remaining whipped cream until completely combined.
4. Pour mousse into serving bowl or individual dessert dishes; cover and refrigerate 8 hours or until set. (Mousse may be refrigerated up to 2 days.)
5. Heat remaining 1/4 cup chocolate chips in small saucepan over very low heat until melted; stir in butter until smooth. Spoon mixture into small resealable food storage bag. Cut small corner off bottom of bag with scissors. Pipe designs on waxed paper-lined plate; refrigerate 15 minutes or until firm. (Designs may be refrigerated up to 3 days.)
6. To complete recipe, carefully peel waxed paper from chocolate designs; place on mousse before serving.