Diplomatic Cake

Method

1. Prepare the sponge cake: In a planetary mixer or a bowl with an electric whisk, add the sugar, eggs, and a pinch of salt. Whip for about 10 minutes until the mixture is well whipped and passes the 'writing test' (when you lift the whisk, the batter writes on itself).
2. Sift the all-purpose flour and potato starch together, then gently fold into the egg mixture from bottom to top using a spatula.
3. Line a rectangular cake pan with parchment paper and pour in the batter. Bake in a static oven preheated to 160°C on the lower rack for about 50 minutes.
4. Prepare the pastry cream: In a saucepan, pour the milk, add the vanilla seeds, and bring to a boil.
5. In a bowl, mix the sugar and egg yolks with a whisk (do not whip). Finally, add the cornstarch.
6. Pour the hot milk into the bowl and mix. Then transfer the mixture to a clean heavy-bottomed saucepan and cook over medium heat, stirring constantly until thickened.
7. Pour the pastry cream into a baking dish, cover with plastic wrap directly on the cream to prevent a skin, and let it cool completely at room temperature, then refrigerate.
8. Prepare the puff pastry: Take the store-bought puff pastry, divide it in half, and roll out one piece to a thickness of 3 mm, forming a rectangle 35x24 cm.
9. Transfer the pastry onto a baking sheet lined with parchment paper. Prick the pastry all over with a fork to prevent excessive rising. Dust with powdered sugar.
10. Bake in a static oven preheated to 175°C for 25-30 minutes. Repeat with the second piece of puff pastry.
11. Prepare the soaking syrup: In a small saucepan, combine water and sugar. Heat until the sugar dissolves completely.
12. Remove from heat, add rum, stir, and let cool completely.
13. Assemble the cake: Stir the chilled pastry cream to soften it. Meanwhile, whip heavy cream with a little powdered sugar until soft peaks form.
14. Gently fold the whipped cream into the pastry cream until smooth.
15. Trim the crust from the cooled sponge cake and trim the edges to make a neat rectangle.
16. Trim the baked puff pastry sheets to the same size as the sponge cake, using the sponge cake as a guide.
17. Place one puff pastry layer on a serving plate with the smooth side up. Pipe a layer of cream onto it.
18. Place the sponge cake on top and brush it generously with the soaking syrup.
19. Spread another layer of cream over the sponge cake.
20. Place the second puff pastry layer upside down on top to achieve a perfectly smooth surface.
21. Dust the top with powdered sugar. Let the cake rest in the refrigerator for at least 1 hour before serving.