You can use any sweet or savoury filling of your choice to make the buns. Some of the options are :
Savoury - cream cheese OR crumbled feta cheese OR crumbled paneer/ cottage cheese, flavoured according to your choice. You can also use any other savoury filling that you want.
Sweet - cream cheese (traditional filling), or any other filing of your choice like chocolate, dried fruit like dates, raisins, chopped nuts, chopped fruit , sweetened coconut filling among others.
Before starting ensure that the melted butter has cooled down a bit before using it to make the buns