In a medium bowl, whisk together egg yolks and cornstarch until smooth. Gradually whisk hot milk mixture into egg yolk mixture until well combined. Pour milk mixture back into saucepan; cook over medium heat, whisking constantly, until mixture simmers. Let simmer for about 2 minutes. Strain mixture through a fine-mesh sieve into a heatproof medium bowl; discard vanilla bean. Stir in butter until melted and well combined. Cover with a piece of plastic wrap, pressing wrap directly onto surface of custard to prevent a skin from forming. Refrigerate until thick and cold, at least 4 hours, or for up to 2 days.

Position oven rack in bottom third of oven. Preheat oven to 350°F (175°C). Grease and line the bottom of an 8-inch (18 to 20 cm) springform pan with parchment paper (do not grease sides of pan).

Fit mixer with beater bowl and balloon whisk attachment. Add egg mixture, and beat at medium speed (3 o’clock) until light and fluffy, 4 to 5 minutes. Add reserved vanilla bean seeds, and beat at low speed (12 o’clock) until combined. Sift flour and cornstarch over mixture; beat at low speed (12 o’clock) until mixture is just combined. Spread batter into prepared pan.

Bake until a wooden pick inserted in center comes out clean, 18 to 25 minutes. Let cool in pan for 10 minutes. Gently run a knife between cake and sides of pan to loosen cake. Remove sides of pan, and invert cake onto a wire rack. Gently remove parchment, and let cool completely. Freeze cooled cake, uncovered, for 1 hour.

Using a serrated knife, carefully cut cold cake horizontally into three layers. Place 1 cake layer on a serving plate; spread jam on top. Place second cake layer on top of jam. Spread cold custard on top. Place remaining cake layer on top of custard; refrigerate cake while whipping cream.

Fit mixer with beater bowl and balloon whisk attachment. Beat cold cream at medium-high speed (5 o’clock) until stiff peaks form. Using an offset spatula, spread a thin layer of whipped cream on top and sides of cake. Spoon remaining whipped cream on top of cake; using an offset spatula, spread cream into a dome shape. Refrigerate cake while rolling marzipan.

Lightly dust a sheet of parchment paper with cornstarch. Place green marzipan on prepared parchment; lightly dust with cornstarch, and top with another sheet of parchment paper. Roll green marzipan between parchment into a 1/8-inch- thick (3mm) circle (about 13 to 14 inches/33 to 36 cm). Remove top sheet of parchment. Gently roll the marzipan onto rolling pin, and transfer to top of cake. Using your hands, even out marzipan to fully cover cake; trim off excess around edges of cake with a sharp knife. If serving immediately, top with marzipan rose, and dust cake with confectioners’ sugar. If not serving immediately, refrigerate cake, uncovered, for up to 24 hours. (See Notes.) Top with marzipan rose and dust with confectioners’ sugar just before serving