Bake the puff pastry sheet in the oven at 410°F (210°C) for 15 minutes.

Separate the egg whites from the yolks. Beat the egg whites until stiff peaks form, then add the vanilla sugar.

Blend the rice pudding until smooth. Once blended, add the egg yolks, vanilla sugar, and butter.

Finally, gently fold in the beaten egg whites. Pour the mixture into the baked puff pastry shell and bake at 350°F (180°C) for 35 minutes.