Preheat the oven to 350°F (175°C). Grease and line a round cake pan (or two smaller pans) so you can slice the baked sponge into three even layers later.
Make the sponge: In a large bowl, beat the Large Egg with Granulated Sugar until pale and ribbon-like. In a separate bowl, sift together the All-Purpose Flour, Potato Starch, Baking Powder and Salt. Fold the dry mixture gently into the egg mixture, alternating with warm Whole Milk and a little melted Unsalted Butter, keeping the batter light and airy. Pour into the prepared pan(s) and bake until springy and a skewer comes out clean. Cool completely, then slice horizontally into three even layers.
Prepare the vanilla pastry cream: Split the Vanilla Bean and scrape the seeds into a saucepan with Whole Milk. Heat gently until just steaming. Whisk together the Egg Yolk with a little Granulated Sugar and the Cornstarch until smooth, then temper with a little of the hot milk. Return the mixture to the saucepan and cook over medium heat, whisking constantly, until thick and glossy. Off the heat, whisk in a knob of Unsalted Butter for sheen and richness.
Stabilize the cream (optional but traditional for easier slicing): Soften the Gelatin Sheets in cold Water for a few minutes, squeeze out excess water and dissolve the bloomed gelatin in a spoonful of the warm pastry cream; then fold that back into the rest of the custard. Cover with plastic touching the surface and chill until cool.
Whip the cream: Beat the Heavy Cream with a little Powdered Sugar to soft peaks. For a lighter, more spreadable filling, fold about two-thirds of the whipped cream into the cooled pastry cream to make a mousse-like filling; reserve the remaining whipped cream to coat the outside of the cake.
Assemble the cake: Place the bottom sponge layer on your serving plate and spread a thin layer of Seedless Raspberry Jam over it. Spoon a third of the custard-cream mixture on top and smooth. Add the second sponge layer, repeat with jam and another third of the cream, then top with the final sponge layer.
Coat and chill: Spread the reserved whipped cream evenly over the top and sides of the assembled cake to create a smooth dome. Chill the cake for at least 30 minutes so the surface firms up for covering.
Cover with marzipan: Lightly dust your work surface with a little Icing Sugar and roll out the Green Marzipan to a circle large enough to cover the cake. Drape the marzipan over the chilled cake and smooth gently down the sides. Trim the excess and neaten the base.
Decorate: Roll a small amount of Pink Marzipan into a rose or rosette and place on top; use any extra green marzipan to make leaves or a border if you like. Dust very lightly with a little extra Icing Sugar or Powdered Sugar for a delicate finish.
Chill before serving: Refrigerate the assembled Prinsesstårta for at least an hour to let flavors meld and the marzipan set. Slice with a sharp knife and serve chilled for best texture and flavor.
