Combine sugar, 1/4 cup water, and the egg yolks in a medium-size metal bowl. Set bowl over saucepan of simmering water (be sure bottom of bowl is not touching water).
Beat yolk mixture with hand mixer set on medium-high about 5 minutes or until mixture registers 140° F on an instant-read thermometer. Continue cooking for 3 minutes, beating constantly.
Cut the vanilla bean in half horizontally, and using a knife score it down the center vertically. Use a small spoon to scrape out the seeds and reserve to the side.
Remove bowl from over water. Add warm, melted white chocolate and vanilla bean seeds. Beat about 5 minutes, until cool.
Beat heavy cream in a large bowl to stiff peaks. Gently mix 1/4 of the whipped cream into cooled white chocolate mixture. Fold white chocolate mixture into remaining whipped cream until no streaks remain. Cover and cool in refrigerator for at least 3 hours.
To serve: Alternate layers of mousse and berries in glasses or bowls, a scant 2/3 cup mousse per serving.