Make the pancakes: Mix thoroughly the flour, egg, sugar, salt and ¾cup milk. Let the batter rest for 1 hour. Make 12pancakes; for method see basic recipe.
Make the filling: Mix ½cup milk, the vanilla sugar and grated lemon rind, and bring to a slow simmer. Add walnuts and raisins and mix well. Turn off heat.
Spread out a large white unstarched napkin or small tablecloth. Butter the center of it, in about a 10-inch circle. Place the pancakes on it, one by one, spreading alittle of the filling on each pancake.
Pick up the four corners of the napkin and pull it around the pancakes to form a balloon, tightly but without breaking the contents. Then tie the napkin with string.
Bring 5cups milk and 2tablespoons butter to a boil in a very large pot. Place the napkin-balloon in it and cook for 15 minutes.
Lift the napkin from the pot, cut the string, and carefully open the package. Put the pancake-balloon on a dish.
Keep the balloon warm: Bring a potful of water to a boil, and place the plate with the balloon on top of the pot. Cover with a large dish. The steam will keep the ballon hot without drying it out.
Make the sauce: Whip eggs, sugar and flour, and little by little add the milk. Add the vanilla bean. Put the sauce in the top part of a double boiler over simmering water and cook, stirring all the while. (Or cook directly over very low heat.) Then turn off heat and mix in the rum. This sauce has a tendency to become thick. If too thick, whip in alittle milk or cream.
Just before serving, pour the sauce over the balloon, and sprinkle the toasted walnuts and almonds over all.
Slice as you would a cake. If you prefer, the sauce can be served separately.