Prepare the orange curd first as it needs to cool. In a large glass mixing bowl, combine the eggs, egg yolks and sugar. Whisk vigorously for 3 to 4 minutes, until the mixture is thick and more pale in color. It should ribbon off the whisk. Pour the mixture into a stainless steel pot and add the orange or mandarin juice; whisk to combine. The juice needs to be pulp-free; strain it ahead of time if necessary. Cook the orange curd over medium-low heat, stirring constantly along the bottom of the pan. Cook for 4 to 5 minutes, until the curd thickens and starts to bubble lightly. Remove from heat immediately. Add the unsalted butter and vanilla and whisk until the butter is completely melted. Cover the curd and place into the refrigerator to cool.
Making the Sponge Cake:
Next, prepare the sponge cake. Preheat the oven to 350F/177C. Prepare a 9 or 10-inch springform pan by lining the bottom with parchment paper; do not grease the sides. (If you are not planning to use an acetate cake collar to assemble the cake, I recommend using a 10-inch pan; otherwise the jello will not fit) Place the eggs, sugar and vanilla extract into a stand mixer bowl, then whisk on high speed for 7 to 9 minutes. The eggs need to be thick, pale in color and super fluffy. In a separate bowl, combine the dry ingredients: flour and baking powder. Sift the dry ingredients into the egg mixture in small increments, folding gently but thoroughly after each addition.
Pour the prepared batter into the springform pan and bake in the preheated oven for 18 to 20 minutes, until the top is a rich, golden color. Remove from the oven and let the cake cool. Once cooled, use a long serrated knife to split the layer in two. (If using 10-inch springform, leave the layer whole and use as the base only) Using a pastry brush or a dispenser bottle, soak the cake layers generously with the sweet wine or a simple syrup; set aside.
Making the Mandarin Mousse:
Next, prepare the stabilized whipped cream but only once the curd has cooled. Pour the chilled heavy cream into a stand mixer bowl and add the confectioner's sugar. Add 3 cups of sugar instead of 2 for a sweeter cake. Whisk the heavy cream for 4 to 5 minutes on medium-high speed until stiff peaks form. Meanwhile, in a small cup or bowl, combine the unflavored gelatin and water. Stir the two together, then heat the gelatin in the microwave for about 45 seconds, stirring every 10 seconds, until the gelatin is dissolved. Immediately pour the hot gelatin into the whipped heavy cream with the mixer running on medium speed and whisk for an additional 30 seconds.
As soon as the cream is ready, pour the chilled orange curd into the bowl and use a spatula to very gently but thoroughly fold the orange curd and stabilized cream together. You need the cream to stay as fluffy as possible. Last, fold in the chopped mandarins.
Assembling the Cake:
To assemble the cake, wash and dry the same springform pan, then line the sides with acetate; optional. Add the first sponge cake layer, then add half the mandarin mousse, followed by the second layer. Add the remaining mousse over the top and use an offset spatula to spread it evenly. Garnish the top with whole mandarin segments. Place the mousse cake into the refrigerator and let it chill for at least an hour before adding the jello topping.
For the jello topping, measure out 1 cup of boiling water and add the orange jello powder. Stir for a few minutes until completely dissolved. To speed up the process, try microwaving the mixture in 30 second intervals, stirring after each one. Once dissolved, cool the mixture down with ice. The jello needs to be cool to touch before pouring over the mousse. Pour the jello over the back of a spoon and over the cake, just enough to cover the mandarin segments.
Carefully place the cake back into the refrigerator and allow the mousse cake to set completely overnight, or for at least 8 hours. When ready to enjoy, release the springform, remove the plastic and transfer onto a cake stand (watch my video to see how it's done!). Keep cake refrigerated when not serving; it can stand at room temperature for 1 hour.