Heat oven to 350°F. Spray bottom only of 9-inch round cake pan with baking spray with flour, then line bottom of pan with cooking parchment paper or waxed paper and spray again.
In 2-quart saucepan, heat butter and bittersweet chocolate over low heat, stirring occasionally, until melted. Remove from heat; stir in liqueur. Cool slightly, stirring occasionally.
Meanwhile, in large bowl, beat eggs, sugar and salt with electric mixer on medium-high speed about 4 minutes or until pale and thickened. Gradually beat in chocolate mixture until blended. Beat in flour, scraping bowl occasionally, just until blended. Pour into pan.
Bake about 30 minutes or until center is set. (Toothpick inserted in center may not come out clean.) Place pan on cooling rack. With sharp knife, loosen cake from side of pan. Cool 30 minutes. Carefully remove cake from pan to cooling rack. Cool completely, about 1 hour.
In 1-quart saucepan, heat whipping cream just to boiling. Remove from heat; stir in white chocolate until melted and mixture is smooth. (If necessary, stir mixture over low heat for a few seconds until chocolate is completely melted and mixture is smooth.) Pour into small bowl; refrigerate 15 minutes or until mixture is thickened and spreadable.
Place cake on serving plate. Spread white chocolate mixture just over top of cake. In small freezer plastic bag, place milk chocolate chips; seal bag slightly. Microwave on High 45 to 60 seconds; squeeze bag until chocolate is smooth. Cut off small corner of bag; squeeze bag to drizzle chocolate over cake.
To serve, cut cake into slices with sharp knife, wiping off knife after each cut. Store covered in refrigerator. If refrigerated, let stand at room temperature 20 minutes before serving.