Preparation

Preheat the oven to 190 °C. Cut the rhubarb to the desired length of the millefeuille. Place it in a baking dish, sprinkle with sugar, and seal tightly with foil so it cooks in its own steam. Bake for 30 minutes, turning once after about 20 minutes if needed. Let cool and reserve the juice.

For the cream, soak the gelatin in cold water. Scrape the seeds from the vanilla pods. Bring the milk, rhubarb juice, vanilla seeds, and pods to a boil, then remove from the heat.

Remove the vanilla pods. In a separate bowl, mix the cornstarch with a splash of the hot milk mixture, then pour it back into the milk while stirring constantly. Add the gelatin and continue stirring over a cold water bath until the mixture begins to thicken and cool to room temperature.

Lightly sweeten the cream and egg white separately, then whip them into whipped cream and stiff peaks. Gently fold both into the mixture and refrigerate for 3 hours.

Meanwhile, cut the puff pastry into rectangles matching the desired size of the millefeuille, bake according to package instructions, and let cool.

Top each pastry rectangle with three pieces of rhubarb, then add a layer of cream. Repeat once, then finish with a final layer of puff pastry and dust with powdered sugar if desired.