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.