Preheat the oven to 350°F (175°C). Lightly grease a 9-inch round springform pan and line it with parchment paper to ensure easy removal of the cake.
Prepare the sponge cake: In a large bowl, beat the eggs and sugar together until pale and thick, about 5 minutes.In another bowl, combine the flour, cornstarch, baking powder, and salt. Carefully incorporate the dry ingredients into the egg mixture, folding gently to maintain the airiness. Pour the batter into the pan and bake for 27-30 minutes or until a toothpick inserted into the center comes out clean. Let cool.
Make the vanilla custard: In a saucepan, whisk the milk, egg yolks, cornstarch, and sugar over medium heat. Stir constantly until the mixture thickens. Remove from heat and stir in the vanilla extract. Let cool completely in the fridge for 2 hours.
Prepare the stabilized whipped cream: Sprinkle gelatin over water and let it sit for a few minutes. Using a mixer, beat the heavy cream until soft peaks begin to form. Gradually add powdered sugar and vanilla extract. Heat the gelatin in the microwave until melted, then slowly add it to the whipped cream. Beat until stiff peaks form. Set aside 1/2 cup for decoration.
Assemble the cake: Cut the cooled sponge cake into three equal layers. Place the first layer on a cake board or serving platter. Spread raspberry jam and then half of the custard. Place the second layer of cake, followed by the remaining custard. Top with whipped cream and shape into a dome. Place the final cake layer on top and press to secure.
Cover with marzipan: Knead the marzipan with green food coloring until it’s soft and evenly colored. Roll it out on a powdered sugar-dusted surface into a large circle. Drape the marzipan over the cake, smoothing out wrinkles and tucking in the edges. Trim excess marzipan from the bottom.
Decorate: Pipe the reserved whipped cream onto the cake, or add icing decorations for a festive touch. Lightly sprinkle powdered sugar on top for a refined, elegant touch.
Chill the cake in the fridge for at least 1 hour before serving. Slice and enjoy!