1. Preheat oven to 350 degrees. I would suggest lining the sides of a 9 inch springform pan with parchment paper that goes about one inch above the sides of the pan. The mousse will come all the way to the top and just a touch above. The parchment paper ensure that the pan sides will be tall enough. It’ll also make it easier to remove the cake from the pan at the end.
2. To make the crust, combine the crust ingredients in a medium sized bowl, then press into the bottom of the springform pan.
3. Bake the crust for 10 minutes, then remove from oven and set aside.
4. To make the white chocolate ganache layer, place the white chocolate chips in a medium sized bowl. Heat the butter, heavy whipped cream and lemon zest in a glass measuring cup. The butter should melt and then the whole mixture should come to a boil. Once boiling, pour the heavy cream mixture over the white chocolate chips, then cover for about 5 minutes.
5. Whisk the white chocolate mixture until smooth. If it doesn’t completely melt, microwave in 10 second increments, whisking in between, until smooth.
6. Spread white chocolate ganache evenly over the crust, then refrigerate until firm.
7. To make the lemon curd, combine all ingredients in a double boiler (or in a metal bowl over a pot of simmering water, like I did). Heat while stirring constantly with a whisk until mixture reaches 160 degrees. Refrigerate until thick and cool.
8. Once lemon curd is cooled and firm, finish the lemon mousse. Sprinkle the powdered gelatin over the cold water in a small bowl. Allow to sit for about 5 minutes, then heat for about 5-10 seconds, whisking until warm and smooth. Set aside to cool to room temperature.
9. In another mixer bowl, whip heavy whipping cream on high until it begins to thicken. Add powdered sugar and continue to whip on high until stiff peaks form, while slowly pouring in the cooled gelatin.
10. Carefully fold the lemon curd into the whipped cream, then spread the lemon mousse evenly over the white chocolate ganache.
11. To make the blueberry mousse, puree blueberries then press through a strainer to remove pulp.
12. Sprinkle the powdered gelatin over the blueberry puree, trying to make sure the powder is all touching the puree. Set aside for about 5 minutes to bloom and firm up.
13. Once gelatin is firm, heat in the microwave for 5-10 seconds, then whisk until melted and smooth. Set aside to cool to room temperature.
14. While gelatin mixture cools, make the whipped cream. Whip heavy whipping cream on high until it begins to thicken. Add powdered sugar and continue to whip on high until stiff peaks form.
15. Once the gelatin is room temperature, fold it into the whipped cream until well combined.
16. Spread the blueberry mousse in an even layer over the lemon mousse layer, then place in the refrigerator.
17. Refrigerate cake until firm, 5-6 hours, then remove from springform pan and place on a serving plate. Top cake with blueberries and lemon slices/zest.
18. Refrigerate until ready to serve.