For Crust: Preheat oven to 350°F. Mix all crumbs in bottom of 9-1/2-inch springform pan. Pour butter over. Mix with fork until well-blended. Press mixture firmly onto bottom of pan. Bake 8 minutes. Cool crust on rack. For Dark Chocolate Mousse: Stir both chocolates in double boiler over barely simmering water until smooth and melted. Cool to lukewarm. Using electric mixer, beat yolks to blend in large bowl. Gradually add sugar and beat until pale yellow and slowly dissolving ribbon forms when beaters are lifted. Blend in Frangelico, then melted chocolate. Whip cream in another bowl until soft peaks form. Fold 1/3 of cream into chocolate mixture. Gently fold in remaining cream. Pour mixture into crust; smooth top. Freeze until set, about 30 minutes. For White Chocolate Mousse: Bring 3/4 cup cream to boil in heavy small saucepan. Reduce heat and simmer 2 minutes. Finely chop white chocolate in processor. Pour hot cream through feed tube and blend until mixture is smooth, about 1 minute. Transfer to medium bowl. Cool completely.

Stir Frangelico into white chocolate mixture. Using electric mixer, whip 1 1/3 cups cream in medium bowl until peaks form. Fold 1/3 cream into white chocolate mixture. Gently fold in remaining cream. Pour over dark chocolate mousse; smooth top. Freeze until top of mousse sets.

Cover mousse and freeze at least 24 hours. Run knife around edges of pan to loosen. Release pan sides from mousse. Smooth sides of mousse with icing spatula. Return to freezer.

Stir 2 ounces of semisweet chocolate and butter in heavy small saucepan over low heat until melted and smooth. Cool to lukewarm. Spoon into parchment cone or pastry bag fitted with small plain tip. Pipe chocolate in decorative pattern around edge of mousse. Freeze until chocolate sets, about 10 minutes. (Can be prepared 2 days ahead. Cover with plastic wrap.) Cut frozen mousse into wedges to serve.