Dissolve yeast in warm water, add flour, salt, mix together in leave in a warm place for half an hour (leave a plate underneath)
Pour sourdough in a large bowl, add butter, sugar, eggs, vanilla, salt and mix. Add flour and mix well till you have a smooth, pleasant to touch dough. Leave for 1-1.5 hours.
In the meantime prepare the filling
Beat well cooled butter with icing sugar. Add vanilla and flour.