Preheat the oven to 180 C.
Beat the butter and sugar until pale and creamy. Add condensed milk and vanilla and mix well. Add flour, baking powder and chocolate, mix to a firm dough. Add a little extra flour if the dough is sticky. Roll into balls and place on a cold greased tray. Press gently with a fork. Bake for 15 minutes until golden. Cool on a cake rack and store in an airtight container once cold.