At least 2 days ahead, combine vanilla bean and sugar in processor or blender and mix until bean is finely ground.
Pass through fine sieve, reserving large pieces of vanilla for another use.
Transfer vanilla sugar to airtight container and cover.
Let stand until ready to use, preferably several days or longer.
Melt white chocolate in small bowl set over hot water, stirring constantly until smooth.
Whisk in butter 1 piece at a time, blending well after each addition.
Beat egg yolks with vanilla sugar until mixture is thick and pale yellow and forms a ribbon when beaters are lifted.
Pour into heavy saucepan or double boiler and whisk over low heat several minutes until very thick.
Whisk in melted white chocolate and continue beating until completely cool.
Beat whipping cream in chilled bowl until stiff.
Beat egg whites until foamy.
Add salt and cream of tartar and beat until stiff but not dry.
Stir 1/4 of this mixture into chocolate mixture to loosen.
Gently fold in remainder until almost incorporated, then fold in whipped cream.
Cover and refrigerate until set, several hours or preferably overnight.
About 15 minutes before serving, spoon mousse into goblets.
Melt dark chocolate and whisk in remaining 2 tablespoons butter.
Carefully spoon thin layer over each mousse and sprinkle minced pistachios in center.