To Make White Chocolate Pastry Cream: In a medium bowl, whisk together egg yolks and sugar until pale yellow. Whisk in cornstarch. In a medium saucepan, heat milk and vanilla bean to just below the boiling point. Slowly whisk a small amount of hot milk into egg yolk mixture, then stir egg yolk mixture back into pan. Bring to a boil over low heat, stirring constantly.

Stir in butter, then strain mixture through a sieve. Place plastic wrap over the surface of mixture to keep a skin from forming. Allow to cool slightly, then stir in melted white chocolate. Chill pastry cream.

Slice bananas and toss with the lemon juice to keep from discoloring. Set aside.

In a large bowl, whip chilled whipping cream until stiff peaks form. Fold into pastry cream, then fold in bananas and liqueurs. Fill tart shell with banana cream filling.

Top generously with white chocolate curls, then dust with cocoa. Serve soon after assembling so crust doesn't get soggy.

To make white chocolate curls, carefully draw a vegetable peeler across the broad, flat surface of a room-temperature (about 80 degrees) chunk of white chocolate to make large, thin curls. Refrigerate until ready to use.