Beat the butter with salt, sugar, vanilla sugar and lime peel until thick and creamy. Mix in the eggs one by one. Mix flour and baking powder and stir in alternately with yogurt and half of the lime juice. Grease a cup cake pan and dust with flour. Pour in the dough and smooth it out. Bake in a hot oven at 175 degrees for about 1 hour. Let it cool down for about 10 minutes and then turn it out of the mold and let it cool down. Mix the powdered sugar with 1-2 tablespoons of lime juice to a smooth glaze. Draw strips over the cake with a spoon and allow to dry.