Preheat and Prepare the Pan:

Heat your oven to 350°F (175°C). Grease a 9-inch round springform pan and line it with parchment paper for easy removal.

Make the Sponge Cake:

In a mixing bowl, beat eggs and sugar on high speed until the mixture becomes pale, thick, and triples in volume. This should take about 5 minutes. The batter should fall in ribbons and leave trails on the surface. In a separate bowl, whisk together the flour (or cornstarch), baking powder, and salt. Gently fold the dry ingredients into the egg mixture until fully combined. Pour the batter into the prepared pan and bake for 27-30 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool slightly in the pan, then remove the springform collar and parchment paper. Transfer the cake to a wire rack to cool completely.

Optional Step: For easier slicing, wrap the cooled cake in plastic wrap and refrigerate overnight. This softens the exterior and allows the cake to stay fresh for up to 5 days.

Prepare the Custard Filling:

In a small saucepan, combine milk, egg yolks, cornstarch, and sugar. Cook over medium heat, whisking constantly, until the mixture thickens. As soon as it thickens, remove it from the heat and whisk vigorously to smooth it out. Stir in vanilla extract, then transfer the custard to a bowl. Cover the surface with plastic wrap to prevent a skin from forming and refrigerate until firm.

Make the Stabilized Whipped Cream:

Sprinkle gelatin over 2 tablespoons of water in a small bowl and let it set. In a stand mixer with a whip attachment, beat heavy cream on medium speed until soft peaks form. Gradually add powdered sugar and vanilla, then increase the speed to high. Microwave the gelatin for 5-7 seconds until liquid, and slowly pour it into the whipped cream while mixing. Continue beating until stiff peaks form. Set aside ½ cup of the whipped cream for later use.

Assemble the Cake Layers:

Using a serrated knife, carefully slice the cake horizontally into three thin layers. Place the bottom layer on a plate or cake board. Spread a layer of raspberry jam, followed by half of the custard filling. Add the second cake layer and spread the remaining custard on top. Pile the whipped cream onto the second layer, shaping it into a dome. Place the final cake layer on top, pressing gently to smooth the edges and create a rounded shape. Cover the entire cake with the reserved ½ cup of whipped cream.

Cover with Marzipan:

Dust your work surface with powdered sugar. Knead the marzipan to soften it, then add leaf green food coloring until the color is even. Roll the marzipan into a large circle, dusting with powdered sugar as needed to prevent sticking. Carefully drape the marzipan over the cake, smoothing it to fit the dome shape. Trim the excess from the bottom using a pizza or pastry wheel, and tuck the edges under the cake with the back of a butter knife.

Decorate the Cake:

Use a soft-bristled brush to paint Wilton candy roses with Baker’s Rose food coloring dissolved in clear extract. Tint leftover marzipan with additional green food coloring, roll it out, and cut small star-shaped flowers using a fondant cutter. Attach the stars to the base of the roses using buttercream or corn syrup. Place the decorated rose in the center of the cake. Dust the top lightly with powdered sugar using a sieve.

Chill and Serve:

Refrigerate the cake for at least an hour, or preferably overnight, to set. Keep it chilled until ready to serve, and store any leftovers in the refrigerator.