Heat the oven to 350°F (175°C). Grease and line a 9-inch round pan.
Whip eggs and sugar until very pale and tripled in volume, 6 to 8 minutes. Fold in sifted flour, potato starch, and baking powder, then fold in melted butter.
Bake for 24 to 28 minutes until the center springs back. Cool completely and split horizontally into three even layers.
Make the Pastry Cream:
Warm the milk until steaming. Whisk yolks, sugar, and cornstarch, then gradually whisk in the hot milk.
Return to the saucepan and cook, whisking, until thick and bubbling. Remove from heat, add vanilla and butter, cover the surface, and chill completely.
Assemble:
Whip cream with powdered sugar to firm peaks. Set the first sponge layer on a plate, spread with jam, then add the second sponge and all the pastry cream.
Top with the final sponge. Spread a thin cream layer over the sides and mound the remaining cream into a smooth dome on top. Chill for 1 hour.
Roll green marzipan into a wide thin circle on powdered sugar. Drape it over the cold cake, smooth gently, trim the base, and decorate with the pink rose. Chill at least 2 hours before slicing.
Notes
Do not overwork the sponge batter after adding flour or it will lose volume.
Ready-colored baking marzipan is easier to roll than soft almond paste.