• Rhubarb
• Clean the rhubarb stalks. Cut them into slices, about 1 cm/ 0.4 inch thick. Place the rhubarb in a bowl, add 130 g/ ⅔ cups sugar, mix well and leave to stand until it releases plenty of juice. I leave it for about 2 hours.

Pie:

• Preheat the oven to 200 degrees Celsius/ 390 degrees Fahrenheit.
• Dough: Place the flour, 50 g/ ¼ cups sugar, the cubed, cold butter, egg, baking powder, and vanilla sugar in the food processor. Pulse until the mixture resembles breadcrumbs.
• Line springform: Turn it onto the working surface and knead very briefly to form a ball. Press the ball into a large circle, place this into the springform (26 cm/ about 10-inch diameter) and press it with your fingers to cover the bottom of the pan. Press some more to form a rim about 4 cm/ 1.5 inch high.
• Filling
• Strain and reserve the juice that the rhubarb released. You should have 500 ml/ 2 cups of rhubarb juice. If you don't have enough, fill the rest with water or some apple juice.
• Mix the vanilla pudding powder with 5 tablespoons of granulated sugar and enough rhubarb juice to form a paste.
• Cook pudding: Place the rest of the juice into a small pan and bring to a boil. Remove from the heat, add the pudding powder-sugar mixture while whisking all the time, place back on the heat source and allow to bubble for a few seconds until the mixture thickens. Keep stirring all the time.
• Combine: Mix the pudding and the rhubarb pieces together and spread this mixture over the pastry in the springform.
• If using cornstarch instead of vanilla pudding powder, cook the same as instructed above, and remember to add 1-2 teaspoons pure vanilla extract (to taste) once the pudding is cooked.
• Bake the pie for 35 minutes before topping it with the meringue.
• Meringue
• Whisk the egg whites: Place into a clean bowl and whisk with an electric mixer until quite stiff. Add the last 130 g/ ⅔ cups granulated sugar gradually and continue whisking until the mixture becomes stiff and glossy.
• Spread the meringue on top of the pie using a spoon and continue baking for another 10 minutes.
• Leave to cool in the springform for at least half an hour; then, you can transfer the pie to a cake plate.