Wash the green chillies and slit them lengthwise but not all the way through (just to the knob on the top).
Put all the green chillies in the butter milk.
Keep the green chillies in butter milk for three days covered, mixing it well for a minute each day.
On the fourth day squeeze the buttermilk from chillies, spread them in a plate and dry in the sun.
In the night return the greenchillies to the buttermilk and cover them.
The next day repeat the same procedure and do this for seven days.
After the final thorough drying (and you may need to sit them out in the sun for 2-3 days bringing them in at night but not putting them in buttermilk-- just sit them on the countertop), they can be stored in an air tight container.