Bake the Cake: Preheat oven to 325 degrees F. You will need two 8” round cake pans, line w/ parchment paper at the bottom, greased and floured.
Into a large mixing bowl, sift together flour, sugar, baking powder, and salt.
Make a wide well in the middle of the dry ingredients. Add in the following order, without mixing, the oil, egg yolks, the lemon zest and passion fruit juice. With a large wire whisk, beat until smooth. Set aside.
In the bowl of an electric mixer, beat the egg whites with the cream of tartar until they hold a stiff peak when the beaters are raised. They should not be beaten until they are dry.
In three additions, fold about 3/4 of the yolk mixture into the whites. Then fold the whites into the remaining yolk mixture. Do not handle any more than necessary.
Gently pour the batter into the pans. Bake for 30 minutes at 325 degrees F, or until the top springs back when lightly pressed with a fingertip. Cool completely.
Make the Mousse: Fill a medium bowl with ice water; set aside. In a small saucepan, combine eggs, sugar and 1/3 cup passion fruit juice. Cook, stirring constantly, over medium-low heat until the mixture thickens enough to coat the back of the spoon, about 3 minutes. Remove from heat, immediately add the butter and stir well to combine.
In a medium saucepan, combine 1/4 cup passion fruit juice and 2 tablespoon cold water. Sprinkle the gelatin over the juice, let stand for 5 minutes. Place over low heat until gelatin is completely dissolved, about 2 minutes.
Pour the passion fruit/egg yolk mixture through a fine sieve into a bowl. Stir in the gelatin mixture. Place the bowl over the ice water, whisk constantly until mixture has cooled.
In a chilled bowl, whip the heavy cream until stiff peaks form, about 3 minutes. Whisk 1/3 of the passion fruit mixture. Then in 2 additions fold the lightened passion fruit mixture into the remaining whipped cream.
For the whipped cream: whip the cream with the powdered sugar with the wire whisk until firm.
To Assemble: Split the cakes in half so that you have 4 1” rounds. Spread the mouse between each layer and chill atleast 1 hour. Frost all sides with the whipped cream, top with the white chocolate curls and mango slices.