Clean, wash and cut the rhubarb into small pieces. Put the fat, salt, vanilla sugar and 150 g sugar into a mixing bowl, whisk with the whisk of the hand mixer until foamy.
Separate two eggs. Stir in 2 eggs and 2 egg yolks one after the other. Mix flour (up to 1 tablespoon) and baking powder and stir in alternately with the milk. Turn rhubarb in the rest of the flour and fold into the dough.
Grease 16 espresso cups (75 ml capacity) and dust with flour. Fill cups to about 3/4 with dough. Place on a baking tray and bake in a preheated oven (electric cooker: 200 °C/ convection oven: not suitable/ gas: level 3) for 20-25 minutes.
Meanwhile beat 2 egg whites until stiff. Finally, pour in 80 g sugar. Pour the beaten egg whites into a piping bag with a perforated spout and spray the muffins with dip caps 5 minutes before the end of the baking time.
Remove from the oven and leave to cool on a grill. Serve dusted with icing sugar.