Add sugar with condensed milk, mix until sugar melts. Add butter and mix until creamy. Don't use electric mixer.
Add butter mix to dry mix. Add vanilla and make soft dough. Divide in half and mix banana mash with one half. If banana dough becomes too soft,add little flour. Keep doughs in fridge for 15 min. Divide doughs in similar small pieces & give cookie shape by pressing between palms. Make butter cookies with butter dough & banana cookies with banana dough.
Place cookies in pre heated oven. On 180c bake for 15 min or until golden brown.
Take out, cool down. Cookies will harden. Banana cookies will be a bit chewy as it's tough to harden real banana but still delicious. If you want crispier,use banana essence instead of banana mash. Store in an airtight jar.