Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).

In a mixer fitted with the dough hook, combine all of the streusel ingredients until coarse crumbs form.

Transfer crumbs to a baking sheet lined with Gefen Parchment Paper and bake for about 20 minutes, until golden. Cool slightly and break up into crumbs.

Place lemon juice, lemon zest, eggs and sugar in a pot for 15 to 20 minutes, stirring constantly. The mixture should thicken during cooking—do not remove from heat until this happens! Remove from heat, add butter, and mix until thoroughly combined.

Allow lemon mixture to cool for about 15 minutes. Strain the mixture and then blend with an immersion blender or food processor, until you have a smooth, velvety cream. Refrigerate for two hours.

Melt the white chocolate. Heat heavy cream together with vanilla bean and lemon zest. Add melted chocolate and mix well.

Remove vanilla bean, if using, cut it open, and scrape the seeds into the mixture. Chill cream in refrigerator for about six hours and then beat until smooth and stiff.

Lay out 20 mousse cups. To assemble, sprinkle streusel crumbs in the bottom of the cups. Pipe lemon cream on top of the crumbs and finish off with the vanilla cream.