1)Dont heat the oven more than 100 degree c.
2)Keep baking the cookies for longer time like 1 to 2 hours till it is crispy.
3)Taste the cookies, if it is chewy then bake for another 15 to 30 mins till crispy.
4)Turn the oven off and let the tray be inside the oven and cool gradually for another 2 hours. It will make it crispy.
5)If you don't have cream of tartar use lemon juice instead.
6)Use powdered sugar for fluffy texture. I powdered normal granulated sugar in blender till fine.
7)Beat the mixture till there is no sugar grits visible.