In a pot, bring the cream to a boil. Pour the hot cream over the white chocolate and let rest 1 minute without stirring. Whisk until smooth. Cover with plastic wrap directly on the surface. Refrigerate for 8 hours or overnight.
Meanwhile, in a small bowl, sprinkle the gelatin over the water. Let bloom for 5 minutes.
In a small pot, bring the lemon juice and ¼ cup (55 g) of the sugar to a boil.
In another bowl, whisk together the eggs with the remaining sugar. Gradually whisk the warm lemon mixture into the egg mixture to temper.
Pour the mixture back into the pot. Cook over low heat, whisking constantly and scraping the bottom and sides of the pot, until the mixture thickens and coats the back of a spoon, about 3 minutes.
Remove from the heat. Whisk in the gelatin until completely dissolved. Let cool, stirring occasionally, until the mixture reaches 99°F (37°C), about 15 minutes. Add the butter all at once. Beat with a hand blender until smooth. Cover with plastic wrap directly on the surface of the cream. Refrigerate for 1 hour.
With the rack in the middle position, preheat the oven to 325°F (165°C). Line the bottom of two 8-inch (20 cm) springform pans with parchment paper. Do not butter the pans.
In a bowl, combine the flour and baking powder. Set aside.
In a large bowl, whisk together the egg yolks with ¾ cup (160 g) of the sugar, the oil, lemon zest and juice, and salt. Set aside.
In a third bowl, beat the egg whites with an electric mixer until soft peaks form. Gradually add the remaining sugar, beating until stiff peaks form.
Add the dry ingredients to the egg yolk mixture to form the batter. Stir one-quarter of the meringue into the batter. With a spatula, gently fold the remaining meringue into the batter. Divide the batter between the prepared pans.
Bake for 35 minutes or until a toothpick inserted into the centre of the cakes comes out clean. Remove from the oven and immediately turn over onto a wire rack to cool completely, about 3 hours. Pass a thin blade between the pan and the cake. Unmould.
In a bowl, whisk the white chocolate and cream mixture until soft peaks form. Add the lemon cream and a few drops of food colouring, if desired. Whisk until stiff peaks form. Transfer 1/3 cup (75 ml) of the cream into a pastry bag fitted with a round tip and set aside until ready to decorate the cake.
Cut each cake in half horizontally to get 4 layers in total.
Place one cake layer on a serving plate. Top with one-quarter of the lemon cream. Repeat with the remaining ingredients, finishing off with a layer of lemon cream. With the pastry bag, garnish the top of the cake with dots of lemon cream, pieces of graham cracker and crunchy meringues. Refrigerate until ready to serve. The cake will keep for 3 days, covered, in the refrigerator.