Mix the two types of flour with salt, sugar and poppy seed. Add the cold butter cut to cubes, and quickly mix it with your hands (or in a food processor if you have one).
Add the egg and the milk, and mix well.
Form a dough and press to a 2-3 cm thick disc. Put to the fridge for at least an hour (or overnight).
In the meantime, prepare the filling: clean and trim the rhubarb, and cut to 1 cm pieces.
Mix the cornflour with 5 tablespoons of water, and set aside.
Put the rhubarb to a medium saucepan with the sugar and 2 tablespoons of water. Heat over medium and keep stiring for about 6-7 minutes. Add the cornflour mixture, and set aside to cool for about 30 minutes.
Preheat oven to 200 C. Roll the dough to 4-6 mm thick and spoon the filling at the middle. Spread with the back of a spoon and leave about 5 cm frame. Fold the frame over the filling.
Put the cake on a baking tray covered with parchment papert and use eggwash over the folded dough. Bake for 40-45 minutes, but lower the heat after 20 minutes to 180 C.
Serve plain or with vanilla ice cream, to salute the elders.