With the rack in the middle position, preheat the oven to 180 °C (350 °F). Line the bottom of a 20-cm (8-inch) in diameter springform pan with parchment paper and butter.
In a bowl, combine the flour, baking powder and salt.
In another bowl, beat the egg whites until soft peaks form. Add half of the sugar gradually and beat until stiff peaks form. Set aside.
In another bowl, combine the remaining sugar, egg yolks, oil, water and vanilla with a whisk. Stir in dry ingredients.
Using a spatula or a whisk, fold one third of the meringue into the batter. Add the rest of the meringue and fold gently. Pour into the pan. Bake for approximately 50 minutes or until a toothpick inserted in centre comes out clean. Cool completely on a rack. Unmould.
In a saucepan off the heat, combine the sugar, flour and cornstarch. Add the eggs and whisk until the mixture is smooth. Add the milk gradually while whisking. Bring to a boil over medium heat, whisking constantly and being careful to scrape the bottom and corners of the pan. Remove the pan from the heat and add the white chocolate. Stir until melted. Transfer to a bowl. To prevent a skin from forming on the surface of the pastry cream, place a piece of plastic wrap directly on the hot cream. Let cool. Refrigerate until completely cooled, about 3 hours.
In a saucepan, combine the rhubarb and sugar. Bring to a boil, stirring frequently. Simmer about 15 minutes until the rhubarb has softened and the compote thickens. Let cool. Cover and refrigerate until completely cooled.
In a saucepan, gently heat the cream and chocolate until the mixture is smooth. Remove pan from heat, add the icing sugar and whisk about 1 minute. Set aside.
On a work surface dusted with icing sugar, roll the almond paste so that it is slightly larger than the diameter of the cake. Using the cake pan (20-cm / 8-inch), cut a circle in the almond paste. Using a brush, remove the excess icing sugar. Set aside.
Remove the crown of the cake so it is level and flat. Cut the cake horizontally in half to obtain two slices. Spread a layer of pastry cream on the base of the cake. Using a spoon, spread the rhubarb compote on top of the pastry cream. Cover with the second slice of cake. Lightly spread ganache on the cake for the almond paste to adhere well. Position the circle of almond paste onto the cake.
Spread the warm ganache over the cake’s surface. It should be slightly runny. If necessary, reheat the ganache.