Mix the butter, sugar, salt, vanilla sugar and the grated zest until frothy. Stir in eggs one at a time. Mix the flour and baking powder and stir in alternately with the buttermilk.
Grease the baking pan and pour in the batter. Bake at 175 ° for about 50 minutes. Let a little cool down in the tin, turn it out and then let it cool down completely.