Whisk butter, sugar, vanilla sugar and salt with the whisk of the hand mixer until creamy white. Stir in the eggs one after the other. Mix flour and baking powder, add in portions and stir in.
While stirring constantly, allow the egg liqueur to run in. Pour the dough into a greased, flour-spread box form (2 litres capacity, 30 cm long). Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 70 minutes.
If necessary, cover the cake after about 50 minutes of baking time. Take it out of the oven, place it on a rack and let it cool down for about 30 minutes. Carefully turn out of the tin. Let it cool down on the grid.
Dust with icing sugar.