Prick the plums several times, place them in a bowl and sprinkle with Armagnac. Leave to soak for about 1 hour. Cream soft fat, sugar, vanilla sugar, salt and lemon peel with the whisks of the hand mixer. Stir in the eggs one after the other. Add milk. Mix flour (up to 1 heaped tablespoon) and baking powder and stir in by the spoonful. Drain the plums, turn them in the remaining flour and carefully fold into the dough.
Put them into a greased, breadcrumbed ring cake tin (23 cm Ø; 21/2 litre capacity) and bake in a preheated oven (electric range: 175 °C/ gas: level 2) for about 1 hour. Leave the cake to cool slightly in the mould. Turn out onto a cake rack and allow to cool completely. Wash the lemon, rub dry and peel into thin strips with a julienne chiseler. Squeeze juice from one half of the lemon and mix with the icing sugar. Spread the ring cake with it and sprinkle thinly with the lemon peel strips. Leave to dry