Beat the sugar, vanilla sugar and butter with the whisk of the hand mixer until very creamy. Stir in the eggs one after the other. Add 1 pinch of salt, rum and vanilla flavouring. Mix flour, cornstarch and baking powder.
Stir in the dough by the spoonful. Immediately place in a greased, flour-spread box form (26 cm long) and bake in a preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for 40-45 minutes.
Remove, allow to cool and dust with icing sugar.