Start with the pastry cream, which you can make the day before.

• Put a little sugar in a saucepan and pour the milk over it. Scrape out the seeds from the vanilla bean and add them, along with the split vanilla bean, to the milk. Adding a little sugar at the start will prevent the cream from sticking to the bottom  of the saucepan
• Heat on medium heat until it starts to simmer without boiling. Remove from heat, strain and leave to infuse for at least 30 min.
• Soak the gelatin sheets in a bowl of cold water for about 10 minutes while you prepare the pastry cream. Leave to swell.
• Whisk the egg yolks with the sugar until pale and creamy. You can use a magic whisk or a regular whisk.
• Add the sifted cornstarch. whisk to smooth and combine. Do not overwhisk.
• Bring the milk to a simmer over medium heat. Remove just before boiling, and pour a little over the egg mixture through a sieve. Mix well to blend the egg mixture with the milk.
• Add the remaining milk, stir and return the mixuture to the saucepan.
• Bring to medium heat, stirring constantly. The pastry cream will thicken gradually. Do not stop cooking just after it has thickened, but let it boil for one more minute. Remove the pastry cream from heat when it starts to form air bubbles. Stop whisking occasionally to check its progress.
• Off the heat, add the cold butter and stir to incorporate it. Once it has melted, add the well-squeezed gelatin.
• Pour the pastry cream in a thin layer into a wide dish and cover with plastic wrap directly on the surface to prevent a skin from forming. You can prepare it the day before for the next day. The next day, you will only need to finish preparing the diplomat cream.
• Whip the cold heavy cream at medium speed at first, and as soon as it is frothy, continue to whip at maximum speed. It should become firm with stiff peaks on the whisk. It shouldn't be too firm, otherwise you'll have difficulty incorporating it into the pastry cream.
• Smooth the pastry cream with an immersion blender or in the bowl of the stand mixer that you have emptied of the whipped cream. You can also use a hand mixer. It should become very smooth with no lumps.
• Add a good spatula of whipped cream to it and mix without precautions. Add a little more whipped cream and whisk until smooth.
• Pour it all back into the mixing bowl over the remaining whipped cream. Gently fold with a spatula from bottom to top to thoroughly combine the pastry cream and whipped cream.
• Your diplomat cream is ready, put it in a piping bag for immediate use, or set aside in the bowl, covered with paper wrap. Preferably use it within 2 hours after preparation.
• It's smoother and you'll create beautiful rosettes. To make pretty decorations with a piping bag, let it firm up a bit in the refrigerator for good stability. Once piped onto cupcakes, for example, or into a entremets, it freezes very well.